Two-stage optical bi-directional transceiver

Description of equivalent WO 2005119954 A1

TWO-STAGE OPTICAL BI-DIRECTIONAL TRANSCEIVER

CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims priority from United States Patent Applications Nos. 60/576,594 filed June 4, 2004, 60/576,595 filed June 4, 2004, and 60/577,604 filed June 8, 2004, which are all incorporated herein by reference.

TECHNICAL FIELD The present invention relates to a two stage optical filter, and in particular to a planar lightwave c ircuit (PLC) optical b i-directional transceiver for u se in fiber-to-the- home (FTTH) optical networks.

BACKGROUND OF THE INVENTION A bi-directional transceiver, e.g. a triplexer or Voice-Data-Video (VDV) processor, serves as an optical gateway from an FTTH optical network into a subscriber's home. A triplexer is an extremely compact and low-cost access device capable of receiving two high-speed channels (e.g. 1490 nm for telephone & internet, and 1550 nm for video), while simultaneously transmitting on a third channel (e.g. 1310 for information out). All these signals are multiplexed onto a single optical fiber for simple installation. For business purposes the video channel can be omitted forming a two channel bi-directional transceiver or biplexer. Alternatively, additional outgoing information channels can be added, as well as additional incoming data channels. Typical biplexer and triplexer requirements present considerable challenges to conventional PLC design techniques. The optical architecture requires that a laser, nominally 1310 nm in wavelength, is coupled to a single-mode fiber for transmitting optical signals from the home. In the other direction on that same fiber, light at wavelengths of nominally 1 490 nm and 1550 nm from outside the home are captured, demultiplexed and directed to optical detectors. The difficulty arises due to the operational passbands at these wavelengths. At the 1310 nm channel, a band of 50 nm to 100 nm is expected, which provides a large margin within which the laser can operate essentially athermally, whereas bands of only 10 nm to 20 nm width are required for the detector channels. Furthermore, the laser diode operates in a single transverse mode, and the common input/output fiber is a single mode fiber; hence, the path followed by the laser channel must be at all points compatible with single-mode optics. In other words the laser channel's path must be reversible. In the prior art, especially those designs using a single d iffractive structure in a P LC, t here i s n o p ractical means o f addressing a wide wavelength range (~ 1250 nm to 1600 nm) with channels having substantially different passbands.

Prior art devices, such as the one disclosed in United States Patent No. 6,493,121 issued December 10, 2002 to Althaus, and illustrated in Figure 1, achieve the functionality of the VDV processor (triplexer 1) using a number of individually crafted thin film filters (TFF) 2a and 2b, placed in specific locations along a collimated beam path. The TFFs 2a and 2b are coupled with discrete lasers 3 and photo-detectors 4a and 4b, and packaged in separate transistor-outline (TO) cans 6 and then individually assembled into one component. An incoming signal with the two incoming channels (1490nm and 1550nm) enter the triplexer 1 via an optical fiber 7. The first channel is demultiplexed by the first TFF 2a and directed to the first photo-detector 4a, and the second channel is demultiplexed by the second TFF 2b and directed to the second photo- detector 4b. The outgoing channel (1310nm) is generated in the laser 3 and output the optical fiber 7 via the first and second TFFs 2a and 2b. Unfortunately, the assembly of such a device is extremely labor intensive, requiring all of the elements to be aligned with very low tolerances.

Attempts to simplify the housing structure and thereby the assembly process are disclosed in United States Patents Nos. 6,731,882 issued May 4, 2004 to Althaus et al, and 6,575,460 issued January 29, 2004 to Melchoir et al. Further advancements, illustrated in Figure 2, involve mounting all of the elements on a semiconductor microbench ensuring repeatable and precise alignment. Unfortunately, all of these solutions still involve the alignment of TFFs with TO cans. An example of a prior art solution without TFFs is disclosed in United States Patent No 6,694,102 issued February 17, 2004 to Baumann et al., which discloses a bi-directional multiplexer utilizing a plurality of Mach-Zehnder interferometers . In optics, a diffraction grating is an array of fine, parallel, equally spaced grooves ("rulings") on a reflecting or transparent substrate, which grooves result in diffractive and mutual interference effects that concentrate reflected or transmitted electromagnetic energy in discrete directions, called "orders, " or "spectral orders. " The groove dimensions and spacings are on the order of the wavelength in question. In the optical regime, in which the use of diffraction gratings is most common, there are many hundreds, or thousands, of grooves per millimeter.

Order zero corresponds to direct transmission or specular reflection. Higher orders result in deviation of the incident beam from the direction predicted by geometric (ray) optics. With a normal angle of incidence, the angle θ, the deviation of the diffracted ray from the direction predicted by geometric optics, is given by the following equation, where m is the spectral order, λ is the wavelength, and d is the spacing between corresponding parts of adjacent grooves:

<img file="WO2005119954A1_D0001.tif" />

Because the angle of deviation of the diffracted beam is wavelength-dependent, a diffraction grating is dispersive , i.e. it separates the incident beam spatially into its constituent wavelength components, producing a spectrum. The spectral orders produced by diffraction gratings may overlap, depending on the spectral content of the incident beam and the number of grooves per unit distance on the grating. The higher the spectral order, the greater the overlap into the next-lower order. Diffraction gratings are often used in monochromators and other optical instruments. By controlling the cross-sectional shape of the grooves, it is possible to concentrate most of the diffracted energy in the order of interest. This technique is called "blazing. " Originally high resolution diffraction gratings were ruled. The construction of high quality ruling engines was a large undertaking. A later photolithographic technique allows gratings to be created from a holographic interference pattern. Holographic gratings have sinusoidal grooves and so are not as bright, but are preferred in monochromators because they lead to a much lower stray light level than blazed gratings. A copying technique allows high quality replicas to be made from master gratings, this helps to lower costs of gratings.

A planar waveguide reflective diffraction grating includes an array of facets arranged in a regular sequence. The performance of a simple diffraction grating is illustrated with reference to Figure 3. An optical beam 11, with a plurality of wavelength channels λj, λ₂, λ₃ ..., enters a diffraction grating 12, with grading pitch A and diffraction order m, at a particular angle of incidence θj_n. The optical beam is then angularly dispersed at an angle θ_out depending upon wavelength and the order, in accordance with the grating equation:

m L = Λ(sin0._n + sin0_o (1)

From the grating equation (1), the condition for the formation of a diffracted order depends on the wavelength _N of the incident light. When considering the formation of a spectrum, it is necessary to know how the angle of diffraction θ_Nout varies with the incident wavelength θ_ιn. Accordingly, by differentiating the equation (1) with respect to θ_Nout. assuming that the angle of incidence θj_n is fixed, the following equation is derived:

/dλ ^~ / c sθ_Nout P⁾

The quantity d0Nout dλ is the change of the diffraction angle θ_Nout corresponding to a small change of wavelength λ, which is known as the angular dispersion of the diffraction grating. The angular dispersion increases as the order m increases, as the grading pitch A decreases, and as the diffraction angle 0_NOUt increases. The linear dispersion of a diffraction grating is the product of this term and the effective focal length of the system. Since light of different wavelengths λ_N are diffracted at different angles Θ_MOU,, each order m is drawn out into a spectrum. The number of orders that can be produced by a given diffraction grating is limited by the grating pitch Λ, because θκ_0ut cannot exceed

90°. The highest order is given by A/λ^. Consequently, a coarse grating (with large A) produces many orders while a fine grating may produce only one or two.

The free spectral range (FSR) of a diffraction grating is defined as the largest bandwidth in a given order which does not overlap the same bandwidth in an adjacent order. The order m is important in determining the free spectral range over which continuous dispersion is obtained. For a given input-grating-output configuration, with the grating operation at a preferred diffraction order m for a preferred wavelength λ, other wavelengths w ill follow t he s ame p ath a t o ther d iffraction o rders. The first o verlap of orders occurs when

m λ_m = ^{(m + 1)}λ. m+l (3)

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Aλ = ^- (5) m + l

A blazed grating is one in which the grooves of the diffraction grating are controlled to form right triangles with a blaze angle w, as shown in Figure 3. The selection of the blaze angle w offers an opportunity to optimize the overall efficiency profile of the diffraction grating, particularly for a given wavelength. Planar waveguide diffraction based devices provide excellent performance in the near-IR (1550 nm) region for Dense Wavelength Division Multiplexing (DWDM). In particular, advancements in Echelle gratings, which usually operate at high diffraction orders (40 to 80), high angles of incidence (approx 60°) and large grading pitches, have lead to large phase differences between interfering paths. Because the size of grating facets scales with the diffraction order, it has long been considered that such large phase differences are a necessity for the reliable manufacturing of diffraction-based planar waveguide devices. Thus, existing devices are limited to operation over small wavelength ranges due to the high diffraction orders required (see equation 5).

Furthermore, for diffraction grating-based devices fabricated in a planar waveguide platform, a common problem encountered in the prior art is polarization dependent loss arising from field exclusion of one polarization caused by the presence of conducting metal S (a reflective coating) adjacent to the reflective facets F.

An optical signal propagating through an optical fiber has an indeterminate polarization state requiring that the (de)multiplexer be substantially polarization insensitive so as to minimize polarization dependent losses. In a reflection grating used near Littrow condition, and blazed near Littrow condition, light of both polarizations reflects equally well from the reflecting facets (F in Fig. 3). However, the metalized sidewall facet S introduces a boundary condition preventing light with polarization parallel to the surface (TM) from existing near the surface. Moreover, light of one polarization will be preferentially absorbed by the metal on the sidewall S, as compared to light of the other polarization. Ultimately, the presence of sidewall metal manifests itself in the device performance as polarization-dependent loss (PDL).

There are numerous methods and apparatus for reducing the polarization sensitivity of diffraction gratings. Chowdhury, in United States Patents Nos. 5,966,483 and 6,097,863 describes a reduction of polarization sensitivity by choosing to reduce the difference b etween first and second diffraction efficiencies o f a wavelength within the transmission bandwidth. This solution can be of limited utility because it requires limitations on election of blaze angles and blaze wavelength.

Sappey et al, in United States Patent No. 6,400,509, teaches that polarization sensitivity can be reduced by including reflective step surfaces and transverse riser surfaces, separated by a flat. This solution is also of limited utility because it requires reflective coating on some of the surfaces but not the others, leading to additional manufacturing steps requiring selective treatment of the reflecting interfaces.

The free spectral range of gratings is proportional to the size of the grating facets. It has long been thought that gratings with a small diffraction order could not be formed reliably by means of photolithographic etching, because low order often implies steps smaller or comparable to the photolithographic resolution. The photolithographic resolution and subsequent processing steps blur and substantially degrade the grating performance. Therefore, the field of etched gratings has for practical reasons limited itself to reasonably large diffraction orders typically in excess of order 10. Devices with orders ranging close to order 1 have long been thought to be impractical to realize.

Other important considerations in the design of a triplexer is the optical isolation of the 1310 nm channel from the 1490 nm and 1550 nm channels, and the insertion loss of each channel, which must be kept at a minimum. This is particularly true for the 1310 nm laser channel, since the coupling of the laser diode to the waveguide chip is a difficult process and requires a relaxed tolerance afforded by the filter loss. Furthermore, a very flat and wide passband is required for all channels.

In the VDV processor, isolation of close to 50 dB is sometimes required between the laser source at 1310 nm and the receiver channels at 1490 and 1550 nm. In a grating- based device the main source of background light arises from scattering from defects on the facet profile. The facets themselves are arranged to create phase coherent interference to disperse and focus light in a wavelength specific manner. Corner rounding between the reflective facet and the non-reflective sidewall will also be periodic, and therefore spatially coherent, but with an inappropriate phase, leading to periodic ghost images with low intensity. Facet roughness will be spatially incoherent, leading to random low-level background light. Thus, if a strong laser signal is incident on a grating and receiver channels are also obtained from that grating, the receiver channels will have a strong background contributed from the laser, at a level typically 30 dB below the strength of the laser. Isolation of- 50 dB is closer to the requirement for a practical VDV processor.

An object of the present invention is to overcome the shortcomings of the prior art by providing a two-stage optical filter planar lightwave circuit bi-directional transceiver with high isolation and low insertion loss.

In a conventional reflective-grating device, the spectrometer output angles are selected to maximize the throughput of the intended wavelengths to the intended locations. Little consideration is given to Littrow radiation that may be quite intense, almost as intense as the intended output emission. In the realm of optical telecommunications, light that returns along an input path can be disastrous to the overall performance of an optical system. Accordingly, reflective grating-based devices may introduce problems to telecommunications systems. As a result, nearly all components for telecommunications have a specification for maximum "Return Loss", or "Back- reflection", which has been particularly difficult to achieve using reflective grating technology, in which the device has a fundamental layout that is, by design, optimized for reflecting high intensities of light directly back towards the input fiber.

Furthermore, if multiple diffraction orders are intended for use, such that the same wavelength emerges from a spectrometer at several different angles, there is the likelihood that the intensity of the secondary diffraction orders may be extremely weak (down to infinitesimal amounts). Therefore products such as integrated demulitiplexer- channel monitors will achieve poor and possibly insufficient responsivity in the secondary diffraction order channels.

Presently, wavelength separating devices used in optical telecommunications systems are ultimately transmissive in nature, e.g. employing arrayed waveguide gratings or thin-film filters, in which there are no strong interferences caused by light rebounding directly backwards from the component.

An object of the present invention is to overcome the shortcomings of the prior art by providing a multiplexer/demultiplexer with input and output ports optimally positioned in accordance with the grating facet diffraction envelope to minimized back reflection to the input ports and maximize output light collected from different diffraction orders.

Ideally MUX/DEMUX systems perform consistently in spite of small fluctuations in laser wavelength, which requires that the MUX/DEMUX be designed with flat passbands in the frequency domain.

Numerous designs exist for both arrayed-waveguide grating (AWG) and echelle- grating etched w aveguide spectrometers, which are used for optical MUX/DEMUX or optical channel monitors/performance monitors (OCM OPM) in the field of optical telecommunications. Conventionally, flat-passband performance of the spectrometer unit is achieved at the expense of higher insertion loss, by degrading the shape of the passband from the ideal narrow-peaked Gaussian bandshapes, which are common to spectrometers in waveguide based devices. The bandshape is degraded by widening the optical aperture at the entrance to or exit of the spectrometer unit, and/or by introducing aberrations, e.g. de-focus, coma, spherical, to the interference element. Even for an ideal design, a flat passband top with sharp band cutoffs will come only at the expense of spectrometer transmission. Furthermore, the passband flattening will not result in temporal narrowing in the existing designs.

With conventional grating-based devices, such as the ones disclosed in United States Patents Nos. 6,298,186 issued October 2, 2001 to Jian-Jun He and 6,188,818 issued February 1 3, 2001 to Han et al, a f lat p assband p erformance can only be achieved by sacrificing transmission at the peak of each channel. Moreover, there is no shortening of impulse in the time domain accompanying the flattening of the passband in the frequency domain.

Accordingly, an object of the present invention is to overcome the shortcomings of the prior art by providing a MUX/DEMUX including a pair of gratings to be used in sequence such that the emission from the first grating achieves a cyclic offset of incidence angle into the second grating of the system.

SUMMARY OF THE INVENTION Accordingly, the present invention relates to a two stage optical filter planar lightwave circuit device for receiving first and second input channels from a system waveguide and for transmitting an output channel onto the system waveguide comprising:

a laser transmitter for transmitting the output channel;

a non-diffractive filter, having a first passband for multiplexing the output channel onto the system waveguide, and for separating the first and second input channels from the output channel; and

a diffraction grating filter for demultiplexing the first and second input channels, each of the first ands second input channels having a second passband n arrower than the first passband.

The diffraction grating filter comprising an input port for receiving the first and second input channels; a diffraction grating receiving the first and second input channels at an incident angle; and first and second output ports for outputting the first and second input channels from the diffraction grating filter, respectively.

The two stage optical filter planar lightwave circuit device further comprising first and second output waveguides optically coupled to the first and second ports, respectively for transmitting the first and second input channels, respectively; and

first and second photo-detectors optically coupled to the first and s econd output ports, respectively, for converting the input channels into electrical signals.

Accordingly, the present i nvention relates to a planar waveguide optical device comprising: an input port for launching an input optical signal, which is comprised of a plurality of optical channels; a reflective waveguide diffraction grating for dispersing the optical signal into a diffraction envelope having a principle diffraction maximum, a plurality of higher order diffraction maxima, and a plurality of diffraction minima therebetween; and a first plurality of output ports outputting said optical channels; wherein said input port is positioned at one of said diffraction minima to limit the amount of light reflected from the reflective waveguide diffraction grating from re- entering the input port.

Another aspect of the present invention relates to a planar waveguide optical device comprising: an input port for launching an input optical signal, which is comprised of a plurality of optical channels; a reflective waveguide diffraction grating for dispersing the optical signal into a diffraction envelope having a principle diffraction maximum, a plurality of higher order diffraction maxima, and a plurality of diffraction minima therebetween; and a first plurality of output ports positioned along the principle diffraction maximum for outputting said optical channels.

Another aspect of the present invention relates to a planar waveguide optical device comprising: an input port for launching an input optical signal, which is comprised of a plurality of optical channels; a reflective waveguide diffraction grating for dispersing the optical signal into a diffraction envelope having a principle diffraction maximum, a plurality of higher order diffraction maxima, and a plurality of diffraction minima therebetween; a first plurality of output ports for outputting said optical channels; and second plurality of output ports positioned along one of the higher order diffraction maxima for outputting light therefrom.

Accordingly, the present invention relates to an optical channel demultiplexer device for separating an input optical signal into a plurality of output channel bands with a given channel spacing comprising: an input port for launching an input optical signal including a plurality of optical channel bands at the given channel spacing; a first optical grating having a first order a first F SR s ubstantially equal to the given channel spacing, for dispersing each optical channel band over substantially a same range of output angles; a second optical grating having a second order and a second FSR for receiving the optical channel bands from the first reflective grating, for directing each wavelength in one of the optical channel bands at a same output angle, and for directing each optical channel band at a different output angle; and a plurality of output ports for outputting a respective one of the plurality of optical channel bands. Another aspect of the present invention relates to an optical channel multiplexer device for combining a plurality of input channel bands with a given channel spacing into a single output signal comprising: a plurality of input ports for inputting a respective one of the plurality of optical channel bands; a first reflective grating having a first FSR and a first order for receiving each of the optical channel bands at different input angles from their respective input ports, and for directing each optical channel band over substantially a same range of output angles; a second reflective grating having a second order and a second FSR substantially equal to the given channel spacing for combining each optical channel band into the output signal; and an output port for outputting the output signal.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:

Figure 1 illustrates a conventional thin film filter based triplexer;

Figure 2 illustrates a conventional thin film filter based triplexer utilizing a semiconductor substrate;

Figure 3 illustrates a conventional reflective diffraction grating;

Figure 4 illustrates a diffraction grating according to the present invention;

Figure 5 illustrates a reflective concave diffraction grating PLC filter a ccording to the present invention;

Figure 6 illustrates a two-stage optical filter according to the present invention;

Figure 7 illustrates an output spectrum from a second stage of the optical filter of Fig. 6; Figure 8 illustrates an output spectrum from a first stage of the optical filter of Fig. 6.

Figure 9 is a plot of output angle vs. frequency for a reflective diffraction grating;

Figure 10 is a top view of a double-grating subtractive-dispersion MUX/DEMUX according to the present invention;

Figure 11 illustrates a plot of input angle vs. frequency and a plot of output angle vs. frequency for the second diffraction grating of the device of Fig. 10;

Figure 12 is a plot of the angle error vs. frequency for the second diffraction grating of the device of Fig. 10;

Figure 13 illustrates an alternative embodiment of an optical device incorporating the planar waveguide reflective diffraction grating according to the present invention with the input waveguide positioned at a minimum of the diffraction envelope;

Figure 14 illustrates a diffraction envelope from the central facet for the device of Figure 13;

Figure 15 illustrates an alternative embodiment of an optical device incorporating the planar waveguide reflective diffraction grating according to the present invention with the input waveguide positioned at a minimum of the diffraction envelope and the first and second sets of output waveguides positioned at maximums of the diffraction envelope;

Figure 16 illustrates a spectrum for a situation in which an input waveguide is located physically near an output waveguides; and

Figure 1 7 illustrates a spectrum for a situation in which an input waveguide h as b een located at a third diffraction envelope minimum.

DETAILED DESCRIPTION

One of the m ajor concerns in the design of planar lightwave circuit (PLC) diffraction gratings is the manufacturability of the reflecting and sidewall facets F and S, respectively. Furthermore, a major limit to the manufacturability of the facets heretofore, has been the photolithographic resolution limitations. Typical photolithographic procedures are limited to resolutions in the range of 0.5 to 1.0 μm, so the minimal requirement to achieve reasonable performance from a grating is that the reflecting facet size F must be larger than this resolution, say 2.5 to 5 μm or more in size.

In Figure 4, the light path is simplified by the assumption that the input and output angles θ_ιn and θ _out. respectively are identical. This assumption is only to simplify the mathematical treatment of the facet geometry. Accordingly:

F » Λcos#_OT ; and (6)

Equation (1) simplifies to <img file="WO2005119954A1_D0003.tif" />

Combining equations 6 and 7 yields

<img file="WO2005119954A1_D0004.tif" />

From Figure 1 :

§ « an0_w (9) F

Historically, incidence and output angles of 45° to 65° have been used inevitably leading to grating facet aspect ratio of F/S to be about 1 (see Figure 3 and Equation 9). At a wavelength of 1550 nm, one finds from equation (6) that facet sizes, for both reflecting F and non-reflecting surfaces S, of 10-17 μm are easily achievable in the prior art, for DWDM applications. This makes grating facets F manufacturable, but at the expense of large non-reflecting facets (or sidewalls) S contributing to the polarization dependent loss. In the prior art, facet size variation is also done by varying the diffraction order m, i.e. adjusting the numerator of equation (8).

Telecommunications networks have evolved from DWDM to CWDM and FTTH networks. The latter two network architectures have channels spanning large wavelength ranges, from - 1250 nm to ~ 1630 nm. These wide ranges cannot be served by a high- diffraction order device, and often require orders as low as 1. Practitioners of the prior art have not been aware of, or taken advantage of equation (8). At low diffraction orders m and operating angles θj_n and θ_out of 45° to 65° the resulting facet size F for a planar waveguide diffraction grating would be too small to be practically manufacturable. Existing planar waveguide diffraction based devices include AWGs and echelle gratings. Both rely on high diffraction orders; the AWGs need high order operation for guide routing reasons, the echelle technique employs high orders to maintain large facet sizes that are more easily manufactured. Hence, prior art has intrinsic limitations in addressing the CWDM or FTTH network architectures in a planar waveguide platform. The present invention recognizes the importance of equation (8), in particular the fact that it is possible to increase the grating facet aspect ratio F/S through angular dependence of the denominator. As the diffraction angle is reduced, the facet size increases linearly with tanθi„. Additionally, inventors recognize that the increase of the facet aspect ratio F/S yields devices with improved polarization dependent loss and larger free spectral range.

For example, in silica-on-silicon, a diffraction order of 5 or less (yielding the smallest practical free spectral range for CWDM or FTTH networks), at a wavelength of 1550 nm, and size of reflecting facet F to exceed 5.0 μm, would require F/S to be increased to more than 3, which can be accomplished by lowering the diffraction angle to about 25°. Thus, the present invention encompasses all planar waveguide diffraction grating designs with the ratio of reflecting to non-reflecting facets ( or sidewalls) of at least 3. Other planar w aveguide m aterials include silica, silicon oxynitride, s ilicon n itride, silicon on insulator, or indium phosphide

The amount of PDL is strongly dependent on the aspect ratio F/S and the length of the non-reflecting facet S. Conventional echelle designs have an aspect ratio of ~ 1, and are strongly subjected to sidewall dependent PDL; however, for F/S in excess of 3, the non- reflecting facets make substantially smaller contribution to the PDL. By further increasing F/S , it is possible to design manufacturable facets with the non-reflecting grating facet sizes S at or smaller than the wavelength of the reflected light, e.g. S≤3000nm, preferably &lt;2500nm, even more preferably &lt;2000nm, and ultimately preferably &lt;1550nm. For such gratings, the interaction length of light with the metallized sidewall is so small that PDL- free operation of the device becomes possible.

Therefore, when we enter a regime in which tan(θ) is small, i.e. to achieve a 1/3 ratio or θ &lt; 25°, we can reduce sidewall dependent PDL.

From a manufacturability standpoint, if reflecting facets F are large, the facets themselves are reproduced faithfully despite photolithographic resolution limits. Small non-reflecting facets S will likely not be reproduced faithfully, and will be slightly rounded, but grating performance i s not affected. Practitioners o f prior art no doubt have realized that the pitch governs dispersion as per equation (1). However, it is quite common to equate the pitch of a grating to the normal distance between reflecting facets (the sidewall S in Fig. 3). With that thinking, a distortion to the sidewall S could be equated with a distortion to the pitch. This is a mistaken conception, and in fact the pitch is given by equation (6). Counter-intuitively, the pitch increases with F, not S. The present inventors recognize this fact and can increase the aspect ratio, i.e. decrease S/F, shown in equation (9) without risk of affecting the pitch. In fact, the fidelity of the grating reproduction is limited not by photolithography but by the accuracy of the features on the mask itself. This limit is several orders of magnitude (100-fold) smaller than the photolithographic resolution.

Combining equation (8) and (9), we find that: mλ ,, .. S « (10) 2

Thus, by choosing a small diffraction order (m= 3, 2 or 1, if necessary) one can nearly eliminate PDL, because the sidewall size S becomes less than the wavelength.

In a preferred embodiment, illustrated in Figures 4 and 5, a dispersive PLC optical filter 19 includes a concave reflective diffraction grating 20 is formed at an edge of a slab waveguide 21 provided in chip 22. An input port is defined by an end of a waveguide 23, which extends from an edge of the chip 22 to the slab waveguide 21 for transmitting an input wavelength division multiplexed (WDM) signal, comprising a plurality of wavelength channels (λj, λ₂, λ₃ ...), thereto. The diffraction grating 20, as defined above with reference to Figure 4, has an aspect ratio (F/S) greater than 5, and a sidewall length S less than or equal to the average wavelength of the wavelength channels (λj, λ₂, λ₃ ...). The input waveguide 23 is positioned to ensure that the incident angle θ;_n is less than 45°, preferably less than 30°,and more preferably less than 15°, and the grating pitch Λ is selected to ensure that the grating 20 provides diffraction in an order of 5 or less. The diffraction grating 20 disperses the input signal into constituent wavelengths and focuses each wavelength channel on a separate output port in the form of an output waveguide 25, the ends of which are disposed along a focal line 26 of the grating 20 defined by a Rowland circle, for transmission back to the edge of the chip 22. The illustrated device could also be used to multiplex several wavelength channels, input the waveguides 25, into a single output signal transmitted out to the edge of the chip 22 via the input waveguide 23. The input and output ports represent positions on the slab waveguide 21 at which light can be launched or captured; however, the ports can be optically coupled with other transmitting devices or simply blocked off.

In a preferred embodiment, illustrated in Figure 3, a concave reflective diffraction grating 10 is formed at an edge of a slab waveguide 11 provided in chip 12. An input port is defined by an end of a waveguide 13, which extends from an edge of the chip 12 to the slab waveguide 11 for transmitting an input wavelength division multiplexed (WDM) signal, comprising a plurality of wavelength channels (λi, λ₂, λ₃ ...), thereto. The diffraction grating 10, as defined above with reference to Figure 2, has an aspect ratio (F/S) greater than 5, and a sidewall length S less than or equal to the average wavelength of the wavelength channels (λ], λ₂, λ₃ ...). The input waveguide 13 is positioned to ensure that the incident angle θj_n is less than 30°, and the grating pitch Λ is selected to ensure that the grating 10 provides diffraction in an order of 5 or less. The diffraction grating 10 disperses the input signal into constituent wavelengths and focuses each wavelength channel o n a s eparate o utput p ort i n the form o f a n output w aveguide 1 5, the ends of which are disposed along a focal line 16 of the grating 10 defined by a Rowland circle, for transmission back to the edge of the chip 12. The illustrated device could also be used to multiplex several wavelength channels, input the waveguides 15, into a single output signal transmitted out to the edge of the chip 12 via the input waveguide 13. The input and output ports represent positions on the slab waveguide 11 at which light can be launched or captured; however, the ports can be optically coupled with other transmitting devices or simply blocked off. Specific examples for operating the aforementioned optical device are:

5° m= l λa_vg = 1550nm 1550nm 1550nm 1550nm

Λ=8892nm 17784nm 26676nm 14828nm

F=8858nm 17716nm 26574nm 14747nm

S=775nm 1550nm 2325nm 1550nm

F/S=11.4 11.4 11.4 9.5

For a biplexer or a triplexer the relevant passbands are 100 nm for the laser, and ~ 20 nm for the detector channels. Such a device would be impractical to implement with a single diffractive structure because the various channels would share a common physical dispersion. Assume that a spectrometer slab region has been chosen such that the smallest reasonable guiding waveguide widths handle the 20 nm passbands at the grating output. The waveguide width necessary for the 100 nm passband channel would be so wide as to support innumerable modes, creating a device with high sensitivity to fabrication tolerances if a reversible path is necessary for this channel.

With reference to Figure 6, the two-stage optical filter according to the present invention includes a non-dispersive filter 31, a dispersive filter 32, a laser source 33, and first and second photo-detectors 34 and 35 formed in a planar lightwave circuit (PLC) chip 36. A single photo-detector 34 can be provided, when one of the detector channels is omitted. Preferably, the non-dispersive filter 31 is a wavelength selective directional coupler, i.e. two parallel waveguides of specific width, spacing and coupling length, which separates the receiver channels from the laser channel. Alternatively, the non-dispersive filter 31 can be a wavelength d ependent modal interference (MMI) filter o r a phase d ependent wavelength splitter, e.g. a Mach Zehnder interferometer designed for splitting wavelength bands. Instead of a single-stage coupler, a multi-stage coupler or MMI can be used, which provides flatter passbands than those commonly produced by s ingle-stage filters, which slightly improves the insertion loss at the outer edges of the channels, where the passbands from the single-stage filters begin to roll off.

The laser source 33 transmits the data channel along waveguide 41 to the non-dispersive filter 31, which multiplexes the data channel onto output waveguide 42. A system waveguide 43, e.g. an optical fiber, is optically coupled to the output waveguide 43 at the edge of the PLC chip 36. A monitor photodiode 46 can be positioned proximate the back facet of the laser source 33; however, the structure of the present invention enables the monitor photodiode 46 to be positioned upstream of the laser source 33 optically coupled thereto via a tap coupler 47, which separates a small portion (2%) of the laser light. Back facet monitors measure the light produced by the laser, but not what is actually coupled to the waveguide 41, i.e. into the PLC chip 36; however, the downstream photodiode 46 is able to directly measure what light has been coupled in the waveguide 41.

The detector channels must pass through both stages of the filter, i.e. the non-dispersive filter 31 and the dispersive filter 32, and are processed by the grating-based dispersive filter 32. Preferably, the dispersive filter 32 is similar the dispersive filter 19, as disclosed with reference to Figure 5, including a concave reflective diffraction grating 50 with a focal line 56, preferably defined by a Rowland circle. As above, a launch waveguide 53 extending between the non-dispersive filter 31 and the dispersive filter 32 is positioned to ensure that the incident angle θj_n is less than 45°, preferably less than 30°,and more preferably less than 15°. Furthermore, the diffraction grating 50 has a pitch Λ selected to ensure that the diffraction grating 50 provides diffraction in an order of 5 or less.

Typical grating-based demultiplexers exhibit relatively sharp passbands that are difficult to make wide and flat, as required for the bi-directional transceiver application. Accordingly, the present invention incorporates multi-mode output waveguides 51 and 52 at output ports along the focal line 56. The multi-mode waveguides 51 and 52 support an innumerable collection of modes, which serves to flatten the spectral response of the grating output, as shown in Figure 7. Alternatively, the first and second output waveguides 51 and 52 include a multimode section adjacent to the first and second ports, respectively, and a single mode section remote therefrom for providing the diffraction grating filter 31 with a flattened spectral response. The waveguides 51 and 52 direct the light from the output ports to the first and second photo-detectors 34 and 35, respectively.

The present invention achieves the varying passbands for the detector and signal channels by incorporating a dual-stage filter, in which the laser channel is separated from the detector channels,^' which are further demultiplexed with a dispersive element of higher resolution. The passband of the laser channel is therefore determined by the first stage of the filter, e.g. the wavelength-selective directional coupler 31 , while the passband of the detector channels is determined predominantly by the second stage of the filter, e.g. grating-based dispersive element 32. The directional coupler 31 can be designed to easily cover a passband of lOOnm, as shown in Figure 8. The detector channels undergo further processing by the grating.

As demonstrated in Figures 7 and 8, narrow transmission passbands are achieved for detector channels, whereas the laser channel is quite broad. The detector channels at 1490 and 1552 nm encounter both stages of the filter, and they are dispersed into narrow bands by the dispersive filter 32. The output waveguides 51 and 52 used in the dispersive filter 32 enable the passbands to be extremely flat and wide across the whole range of interest. The 1 310 nm radiation is extracted following only the first stage of the filter, e .g. the wavelength-selective directional coupler, with extremely low loss. The loss for the laser channel is therefore far superior to other Triplexer filters in which the laser channel must pass through one or several grating-based elements. The present two-stage configuration ensures that there is no direct path from the laser source 33 to the first and second photo- detectors 34 and 35, and the two channels are always counter-propagating, resulting in extremely high isolation of the laser source 33 from the first and second photo-detectors 34 and 35. The level of isolation is significantly improved from the typical level of 30 dB from a standard grating, and can exceed the 50 dB specification required by some customers.

Rearranging equation (1) to yield the output angle versus the optical frequency, produces: sin/9 = — - sin/9 (11) fnK With reference to Figure 9, the output angle varies in a smooth monotonic manner with respect to optical frequency. If the diffraction grating is designed for sharp imaging, and the input and output apertures are sharply defined, then the optical passband shape for this grating device will be a n arrow p assband shape, with virtually no insertion loss at the peak. In traditional designs, the passband is widened by deforming the grating or widening the optical apertures, such that as the frequency is swept, the response over the output aperture is dulled. The result can be a flat, and potentially sharp-sided passband, at the expense of insertion loss at the peak.

It can be seen from Equation 11 that for a given optical frequency, the output angle can be made to vary by changing the input angle. In fact this is an element of coarse/fine refractive index error correction for standard echelle-grating based optical DEMUX's and OCM/OPMs. Also, from Equation 1, for a given (fixed) output angle, the optical frequency (or wavelength) can be made to vary with the input angle.

Typically, as the optical frequency varies over the passband of a ITU-grid channel, normally the output angle of the light would vary (as in Figure 9), and the light would sweep past the output waveguide. However, if the input angle could be made to vary in a complementary direction, i.e. introduce some frequency insensitivity, then the output angle could be held fixed in place. To be useful as a MUX/DEMUX, by the time the next frequency on the ITU grid is tuned, the light must image onto the next output waveguide, with the same insensitivity to frequency variation over the new passband.

In accordance with the present invention an angular dependence versus frequency is introduced, as in Figure 9, but with a pattern that repeats with a controlled period, e.g. every 1 00 GHz as with the ITU grid spacing. To accomplish this a second diffraction grating is inserted prior to the first diffraction grating of Figure 9, having a Free-Spectral- Range (FSR) of the required period, e.g. 100 GHz, with a geometry chosen to achieve the required angular variations.

Recasting equation (1) in terms of frequency, and subtracting the frequencies of consecutive diffraction orders for the same input/output angle combination, the difference being a constant frequency (disregarding index variations with optical frequency), which is the FSR of the grating. mc / = ^• (12) "^Λ(^sin&lt; ,„^{+ sin}ι ₀J f - f = FSR = —j - , = ^ (13) «Λ sin β + sin β ) m

The required diffraction order for a given FSR is then given by m = - - (14) SR

For a FSR of 100 GHz and a central frequency f of 194.0 THz, the required order i s m=1940. Index dispersion of the waveguide material will result in a slight error in the FSR as the frequency deviates substantially from the point at which the FSR calculation was performed. This can easily be compensated by a slight adjustment to the diffraction order.

For a similar geometry, the grating facet size will scale as the order. Whereas standard DEMUX's in low diffraction orders (m~20) have facets of ~ 10 μm in size, the high order grating will have a facet of ~ 1 mm in size.

To understand how a frequency insensitive design might work, imagine a high order (FSR=100 GHz) grating spectrometer with a Rowland Circle geometry. For convenience of calculation, the output angle of the high-order spectrometer is chosen to be the same as the input angle used in the standard order (m~20) design. We place the Rowland circle of the high-order spectrometer such that the output of this spectrometer is located at the input of a standard spectrometer. The gratings and the input to the high-order spectrometer are arranged such that the coupling of light from the high-order spectrometer to the standard spectrometer i s optimum. The choice of input and output angles, and the grating geometries are for convenience of calculation only.

With reference to Figure 10, a WDM optical signal comprising a plurality of optical channel bands is input an optical waveguide 109 at the edge of a planar lightwave circuit chip 110 and enters a first slab waveguide 111 at input port 112. A first concave reflective grating 113 has a relatively high order, e.g. greater than 1000, preferably greater than 1500, and even more preferably greater than 1800, and a relatively small FSR, e.g. substantially the same as the channel spacing of the optical channel bands to be output. Due to the small FSR, the first grating 113 disperses each channel band over the same small range of output angles through an aperture 114 into a second slab waveguide 116. A second concave reflective grating 117 is positioned at one side of the second slab waveguide 116 opposite the first reflective grating 113 in a face-to face relationship. The first and second reflective gratings 113 and 117 have optical power and focus the light along the same line defined by a Rowland Circle 118. The second reflective grating 117 has a much lower order than the first reflective grating 113, e.g. less than 100, preferably less than 50 and even more preferably less than 25, a much higher FSR, e.g. 10 times greater than the FSR of the first grating, and is designed to convert the small range of input angles (corresponding to the small range of output angles from the first grating 113) into a single output angle for each channel band, i.e. for the small range of wavelengths the output angle of the second grating 117 remains the same. Accordingly, each wavelength in the band of wavelengths in a single channel will be directed at exactly the same spot on an output port, e.g. output port 119a, corresponding to an output waveguide, e.g. output waveguide 120a. When the next channel band hits the second grating 117, the frequency has increased, but the input angle returns to the lower end of the range, resulting in the output angle of the second grating 117 changing. The new output angle from the second grating 117 will remain fixed for all wavelengths in the new channel band, which is output a second output port, e.g. output port 119b. Other waveguides, such as optical fibers, are attached to the edge of the planar lightwave circuit chip 110 for transmitting the optical signals.

The device can also be used in a reciprocal fashion for multiplexing a plurality of input optical channel bands into a single output signal. In this case the second reflective grating 117 r eceives e ach c hannel b and at a d ifferent i nput angle, w hich the s econd reflective grating 117 converts into the same small range of output angles for transmission through the aperture 114. The first reflective grating 113 then converts the small range of input angles into a single output angle, thereby combining all of the channels onto a single output waveguide 109.

In this double-grating configuration, as the input frequency tunes, the output angle of the first spectrometer will vary in a cyclic pattern according to a desired channel spacing of the second grating, i.e. the input signal and the output signals. If the geometry and facet spacing of the first spectrometer are chosen properly, the pattern will repeat every 100 GHz (or other desired channel spacing), with a variation in output angle that becomes a variation in input angle to the second spectrometer. The input angle variation with optical frequency provides a constant output angle for all wavelengths in the band of wavelengths in each channel, which can nearly exactly pin the output image to the designated output waveguide. With reference to equation 11, the second grating 117 is designed so that the change in input angle θj_n compensates for the change in frequency f providing a constant output angle θ_out over the given range of wavelengths in the channel band. For the next channel band, the frequency keeps increasing, but the input angle θj_n reverts back to the lower end of the repeating range, which results in a new θ_out for the next channel.

In actual fact, due to the index dispersion of silica, i.e. the index varies with optical frequency, the output from the first spectrometer 113 is not exactly cyclic at a 100 GHz period resulting in a gradual drift in the output angle as the frequency is tuned over the entire ITU grid. The walk-off can be partially compensated for by re-positioning the output ports 119a and 119b of the second spectrometer 117 relative to their usual positions for a fixed input aperture location. Furthermore, as explained previously, a modification can be made to the diffraction order of the first grating 7 in order to tune its period to the required value.

Figure 11 illustrates the near-cyclic behavior of the input angle θi„ to the second spectrometer 117, i.e. the output angle θ_out of the first spectrometer 113, versus frequency, as well as the stepped behavior of the output angle θ_out of the second spectrometer 117 versus optical frequency. A refractive index dispersion of

« = 1.452061 - 1.342485 l 0^"5(^ - 1545) where λ is stated in nanometers is used for these calculations.

Figure 11 graphically relates the input angles to the second grating 117, and the output angles from the second grating 117, as a function of optical frequency, illustrating the cyclic nature of the input angles, and the resulting stepped response of the output angle. The slight wavelength dependence of the refractive index (of the silica waveguides) leads to a barely perceptible shift in mean input angle to the second spectrometer 117 over the wide frequency range of the C-band; however, in general, the output angles of the second spectrometer 117 do show the expected stepped performance, i.e. over large fractions of each ITU grid the steps show little slope. The angular content of typical waveguide modes in a silica-on-silicon design will have a magnitude on the order of a few degrees. If the angle of coupling into these output guides can be held fixed to a small fraction of this mode angular content, the coupling should remain unchanged.

The graph in Figure 12 illustrates the deviation of the output angles from their mean position across the grid. As can be seen from the figure, the output angles are indeed pinned to their required mean position to within 2 m illi-degrees. The physical spacing between the output guide is approximately 15 μm, so the physical error in the output position from the second grating will correspond to ~ 0.3 μm.

The double-grating subtractive-dispersion design according to the present invention has benefits in the time domain as well as the frequency domain. However, in a standard single-grating design with well optimized sharp (Gaussian) passbands, improved performance will be limited when transformed between the time and frequency domains. A temporal impulse broadening arises because the optical path from the input to any output v ia the n ear e dge o f t he g rating v ersus the p ath v ia t he far e dge o f the g rating differs by a non-zero length, which is indicative of the impulse broadening. As stated above, flat-top passbands are usually obtained by introducing aberrations to the grating or by increasing the input or output apertures; however, none of these solutions reduces the spread in time for different paths across the grating, i.e. the standard flat-top design does not narrow the temporal response. In the double-grating subtractive-dispersion configuration according to the present invention, a ray which follows a short path off the first grating 113 to the input of the second grating 117, will then follow a long path off the second grating 117 to the output port 119a of the second grating. The reverse holds for rays initially taking a long path from the first grating 113. As a result, temporal compression is achieved at the same time as frequency-domain broadening. Accordingly, a subtractive dispersion double-grating device can be utilized at significantly higher data bit rates than a standard design flat passband device. The device illustrated in Figure 10 with two gratings each operating in a Rowland circle geometry was a first embodiment provided as an example for simplicity of calculation; however, there are a few other options that may be more favorable. One option is to design the first grating with a shape that is more appropriate for imaging along a chord at the second grating's Rowland circle centered for the input to the second grating. A second option is to create the first grating to collimate its diffracted light, i.e. imaging to infinity, and the second grating would be shaped in the same manner to re-focus its diffracted light.

The output of the first grating will need to be collected efficiently by the second grating. At the same time some form of aperturing will be needed between the first and second gratings because there will be light from multiple orders emanating from the first grating near the intended input to the second grating. Much of the aperturing will be accomplished simply by the fact that the large-order grating facets are physically quite large, leading to a narrowing of the diffraction envelope from the first grating. If blazed properly, only the intended diffraction orders should arrive with any reasonable intensity onto the second grating. In order to prevent order overlap from confounding the spectrum of the second grating, aperturing would also be needed to restrict the angular range, which enters the second grating from the first one.

A subtractive dispersion spectrometer pair can also be designed using AWGs; however, in this case the high-FSR first spectrometer will have many drawbacks in terms of phase control. For etched grating based devices, facet shaping is a parameter that has no direct analog for AWGs, i.e. straight, circular, parabolic, elliptical, or other facet shapes can be implemented to control the phase of the radiation as it emerges from the high-FSR first grating.

The overall transmission of the double grating device, i.e. the height of any passband, can be quite high. Diffraction gratings that are designed to be astigmatic over limited angular regions and blazed for that region can be as efficient as ~ 0.5 dB excess loss. A theoretical insertion loss of- ldB is not unexpected for the grating-pair device. Traditional channel flattening techniques often require over twice that loss to achieve much less-optimal performance. The sharpness of the passband, i.e. the steepness of the band walls, can be increased by narrowing the frequency span covered by a given optical waveguide mode width. One simple means to do this is to increase the diameter of the Rowland circle of the second grating system, or more generally to increase the physical dispersion of the second grating system. The first grating system would have to be altered appropriately as well. The width of the passband will be limited only by the aperturing that has just been mentioned. For a 40 channel, 100 GHz design, widths of ~ 40 - 50 GHz should be achievable. Depending on the steepness of the walls, these numbers could represent -0.5 dB, 1 dB and -3 dB widths all within a few GHz of each other.

The performance of the double-grating configuration is expected to be near transform- limited, thereby providing good optical performance at higher bit-rates than standard flat- top designs allow.

The present invention can be used to create high-transmission, ultra-flat ultra-sharp passband, high bit rate compatible MUX/DEMUX' s. The present invention could be applied to DWDM, CWDM, 1310/1550 nm splitters, comb filters or optical channel monitors, all by proper choice of diffraction order for the first and second gratings.

The efficiency of the diffraction from a grating is a coherent superposition of the diffraction envelope from individual facets. The positioning of the multitude of facets dominates the mode shape of the emissions from the gratings at specific wavelengths, while the size of the individual facets dominates the relative intensity of different modes at different angles/wavelengths. This diffraction envelope is essentially a (sin(x)/x)² intensity distribution. By carefully choosing the location of the input to the spectrometer at a minimum of the d iffraction envelope, and the required outputs centered about the maximum of the distribution, it should be possible to have optimum transmission to the output of the spectrometer with minimal reflection of light towards the input of the spectrometer.

If secondary diffraction orders are employed as well as the primary orders, then it would be desirable to place the secondary outputs at other maxima of the diffraction envelope, which would improve the signal captured at the secondary outputs, while at the same time reducing the sensitivity of the secondary signal strength to slight changes in grating facet orientation. The intentional utilization of minima and secondary (or higher) maxima of the grating facet d iffraction e nvelope is new. A design u sing these minima or maxima explicitly positions inputs and outputs of the grating spectrometer by accounting for the performance of the grating as a whole.

With reference to Figure 13, a simple optical demultiplexer is designed with one input channel 221 and four output channels 222a to 222d for Coarse Wavelength Division Multiplexing (CWDM). An optical signal with a plurality of optical channels, defined by center wavelengths λj to λ-_t, is launched via input channel 221 into a slab waveguide region 223 to be incident on a grating 224. The grating 224 disperses the optical channels according to wavelength, whereby each optical channel λj to λ₄ is captured by one of the output channels 222a to 222d.

A diffraction envelope from the central facet for the device of Figure 13 is displayed in Figure 14. Note the high principle maximum 231 and the multiple higher-order maxima 232 with minima 233 between them. By moving the input guide 221 to a minimum 233 of the diffraction envelope the return light intensity is greatly reduced. Moreover, by moving the output channels 222a t o 222d to the principle maximum 231 or at least a higher order maximum 232 the transmitted light is maximized. Obviously, positioning both the input guide 221 and the output guides 222a to 222d in minima 233 and maximum 231 is preferred.

With reference to Figure 15, since the position of the diffraction envelope, the design of grating 241, and the position of the input port 242 are all inter-related, the design of a demultiplexer device 240 is an iterative process starting with the design of the grating 241 to generally provide a diffraction envelope with a sufficient amount of higher order minima and maxima. Preferably, the grating 241 is a concave reflective grating, as disclosed above with reference to Figures 4 and 5 , with a focal line along a Rowland circle 243. Next, an initial trial position for the input port 242 is selected, and the resulting diffraction envelope is examined. Assuming the input port 242 was not positioned correctly in the desired higher-order minima, a second trial position is selected. The process continues until the input port 242 matches the desired higher-order minima. Now the primary output ports 244, e.g. Order n, can be chosen based on the primary order maximum, and the secondary output ports 246, which are optically coupled to output waveguides 247, e.g. for optical channel monitoring detector array 248 made up of photo detectors, are selected based on the position of the higher order maximums, e.g. Order n- 1. Ideally all of the output ports 244 and 246 are positioned along the focal line 243 of the grating 241, defined by a Rowland Circle.

Figure 16 illustrates a spectrum for a situation in which the input port 242 is located physically near the output ports 244, such that the input port 242 falls somewhere within the principle diffraction maximum, which is very typical for demultiplexers based on Echelle Gratings. The intensity 251 of the return light signal is comparable to the intensity 252 of the main output signals, and can result in very high Return Loss that is unacceptable for telecommunications-grade optical components. A similar spectrum is calculated and illustrated in Figure 17, in which the input port 242 has been located at the third diffraction envelope minimum. Note the nearly 240 dB reduction o f the intensity 253 of light returning along the input channel.

The sharp dip in the middle of the input channel is a result of the diffraction envelope dramatically minimizing within the span of the input port 242 itself.

The secondary set of output ports 246 are located at higher-order diffraction envelope maxima 232, see Fig. 14, to capture duplicate signals in parallel with the capture of light into the primary output waveguides 244a via primary output ports 244, which is useful, inter alia, as an integrated Demultiplexer/Optical Channel Monitor. In this case, the primary Demultiplexer output ports 244 would fit in the region of the principle diffraction envelope maximum 231, while the secondary output ports 246, e.g. channel monitor guides, for the same wavelengths but at a different diffraction order off the grating 241, would fit in the region of a secondary or higher diffraction envelope maximum 232. Accordingly, monitoring of the optical power in each channel λj to λ₄ of the Demultiplexer can be performed by measurement of the light coupled into a different order, instead of through the insertion of a tap coupler and the subsequent demultiplexing/monitoring of that light signal.