Laser module with external cavity and optical fibre reflector

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a light source for an optical communication system, and more particularly to a laser having an external cavity created by an optical fiber Bragg grating. Such devices are known in the art as HDBR (Hybrid Distributed Bragg Reflector) lasers or Fiber Grating (FG) lasers.

【0002】

[Conventional technology]

Generally, such a device includes a Fabry-Perot semiconductor active element (laser diode) with a terminal facing set that supports an antireflection coating, the active element in the length direction of an optical fiber with one end tapered. Connected to the written grating, the grating is placed at the tapered end adjacent to the facet that supports the antireflection coating. As described in the literature, this type of laser is used as a light source for pump radiation, a single mode light source for optical communication systems, in particular a wavelength split multiplexing system, a mode lock laser for pulse generation within a wide frequency range, etc. Fits very well to do. To get an overview of such laser applications, see, for example, the article "Lightwave Applications of Fiber Bragg Gratings" by CRGiles, Journal of Lightwave Technology, Vol.15, No.8, August 1997. , 1391 et seq., And J. Archambault and SG Grubb in the same book. See "Fiber Gratings in Lasers and Amplifiers", p. 1379 et seq.

【0003】

It is well known that some properties of the above lasers are related to the overall length of the cavity. In the case of a hybrid laser, the total length of the cavity is the cavity length of the active element, the distance between the antireflection facet and the tapered fiber tip, and finally the fiber portion between the tip and the equivalent mirror surface of the grating. Given by the sum with the length of. As is well known, the equivalent mirror surface is to allow the pulse sent by the light source and reflected by the mirror to return to the light source at the same time as the pulse sent into the grating returns. The surface on which the mirror must be placed. In particular, the shorter the laser cavity, the wider the resulting modulation band and the better the mode separation. It is clear that good characteristics regarding modulation band and mode separation are particularly interesting when using a laser as a light source for communication systems. The conventional low-reflection Bragg grating (having an output reflectance on the order of 70%) currently used to form the outer cavity of a hybrid laser has a refractive index symmetrical with respect to the center point of the grating. It has a modulation profile and thus the equivalent mirror surface is substantially centered on the grating. High-reflection gratings, on the other hand, which have substantially 100% reflectance, cannot be used for the outer cavity of a laser, even if they themselves have an equivalent mirror surface offset to one end. This is because they do not allow sufficient power in the fiber.

【0004】

[Problems to be Solved by the Invention]

The gratings used in these applications have lengths on the order of centimeters, so the length of the outer cavity constitutes almost all of the total cavity length. This is because active devices have a cavity length on the order of 200 μm. With conventional gratings, the cavity length cannot be shortened enough to obtain satisfactory properties for the laser. It is conceivable to reduce this drawback by writing a grating at the end of the fiber, but doing so creates another problem when the fiber is secured by the resin on the support of the module. One of the fixation points must correspond to the end to ensure a steady alignment between the active element and the fiber, and under such conditions it is clear that the resin interacts with the grating. .. Experience has shown that the resin modifies the structure of the grating as it cures, thus making this solution infeasible.

【0005】

[Means for solving problems]

These problems are solved by the laser according to the present invention. The outer cavity utilizes a fiber grating having a refractive index modulation profile that reduces the equivalent grating length while maintaining low reflectance. In this way, the overall length of the cavity is also sufficiently limited.

【0006】

BEST MODE FOR CARRYING OUT THE INVENTION

More specifically, the provided laser module includes a Fabry-Perot cavity active element with a facet treated with an antireflection coating and an external cavity made by a low reflectance fiber optic grating. This grating exhibits a non-uniform and asymmetric index modulation profile in the length direction. The refractive index modulation profile is such that the position of the equivalent mirror surface is offset toward one end of the grating, and the grating is provided such that the end is the grating end closest to the laser. In a preferred embodiment, this modulation profile is represented by a curve that has a minimum value of substantially zero and gradually increases monotonically to the maximum value. This minimum value corresponds to the farther edge of the grating and has a substantially horizontal tangent, and the maximum value reaches corresponding to the other end of the grating and is substantially horizontal. It has a tangent, where the curve returns to its minimum with a substantially vertical gradient. As a non-limiting example, the index of refraction modulation profile in the gradual change is the following function: y = exp (-x)²), Y = sin²It can be represented by one of x and y = tanh x.

【0007】

[Example]

As shown in FIG. 1, the hybrid laser, which is shown in its entirety by reference numeral 1, is made of a semiconductor material and has a Fabry-Perot cavity active element 2 having a terminal facet 3 that supports an antireflection coating, and a grating. Includes a constant length optical fiber 4 to which 5 is written. As an example, the active element 2 can be an InGaAsP: InP, SI-BH (Semi-Insulating Buried Heterostructure) laser having a modulation band on the order of 10 GHz or higher. The fiber 4 terminates at a tapered end 6 located adjacent to the facet 3 of the active element 2. In a completely conventional fashion, the active element 2 and the fiber 4 are secured on a common support 7 (eg, belonging to the module package) by a resin (especially acrylic resin) as shown in 8. To ensure that the fiber 4 is properly aligned with the terminal facet 3 of the active element 2, the resin must adhere to the entire end of the fiber 4 including the tapered end 6. However, for the reasons described in the introductory part of this specification, the grating must be outside the fixed area.

【0008】

In fixing the fibers on the support 7, it should be taken into account that environmental conditions and mechanical stresses can fluctuate the pitch of the grating, which can affect the emission wavelength of the module. Such fluctuations can occur, especially due to deformation of the support 7 due to temperature changes or simply the tensile force applied to the fibertail 4'. To avoid such fluctuations in the grating pitch, the fiber 4 is further fixed corresponding to the edge of the support (end wall of the package) as indicated by 8', and the grating area is in the grating area itself. Any stress is bent slightly to dampen it. As mentioned above, the end of the fiber 4 adjacent to the taper 6 cannot be used to write the grating. Therefore, the fiber of the entire portion extending between the end of the tapered portion and the equivalent mirror surface of the grating 5 contributes to the overall length of the laser cavity. In order to shorten the overall length of the outer cavity, the grating 5 used in the laser according to the present invention is a grating having an equivalent mirror surface offset toward the end closer to the active element.

【0009】

FIG. 2 shows an example of a refractive index modulation profile that allows the equivalent mirror plane to shift towards one grating end. This profile is an asymmetric, non-uniform profile that has a virtually zero minimum and gradually monotonously increases until it reaches the maximum, reaching a virtually zero value on a substantially vertical gradient. It is represented by a back curve. This minimum value corresponds to the grating end farther from the taper of the fiber and has a horizontal tangent, and the maximum value reaches corresponding to the grating end closer to the taper, which is also substantially horizontal. Has a tangent line. In this figure, the pitch of the grating 5 is exaggerated and longer than it actually is in order to clarify the drawing. A refractive index modulation profile that meets the requirements of the present invention is, for example, a Gaussian curve, i.e. y = exp (-x).²) Type curve, or y = sin²It corresponds to an x-type curve or half of a y = tanh x-type curve. FIG. 2 shows the case of a half Gaussian profile. A method capable of producing this type of grating is the subject of a patent application (name) "Fiber Bragg Grating with Offset Equivalent Mirror Surface and Its Manufacturing Method" filed by the same applicant at the same time.

【0010】

"Diode lasers and photonic integrated circuits" by LA Coldren and SW Corzine, Wiley & As can be easily seen from the description of Sons, 1995, p. 85 et seq., And as confirmed by the measurements made, this type of profile points the equivalent mirror plane towards one end of the grating. Actually move. Considering a semi-Gaussian profile as shown in Figure 2 as an example, the grating on the order of about 1 cm in length and 70% reflectance (a common value for these applications) is about 2 mm. Has an equivalent length of. By comparison, conventional gratings of the same length and similar reflectance have an equivalent length of about 5 mm. Considering that the fiber ends left available for fixation can be on the order of 3 mm, the present invention provides a laser cavity in an amount that actually corresponds to the fiber portion that cannot be used due to problems inherent in resin fixation. Can be shortened. Those skilled in the art can evaluate the effect of such shortening of cavity length on bandwidth and mode separation by applying well-known relationships.

【0011】

The performance of the laser of the present invention can be evaluated from the graphs of FIGS. 3 to 5. In particular, FIG. 3 shows the power-current characteristics at the fiber output, showing that the present invention has good linearity. Figure 4 shows the amplitude-wavelength characteristics, showing good single-mode performance of the laser. Finally, FIG. 5 shows that a very wide modulation band can be obtained by the present invention. It is clear that the above description is given merely as a non-limiting example and that it can be modified or modified without departing from the scope of the invention.

[Simple explanation of drawings]

[Figure 1]

It is a schematic diagram of a hybrid laser.

[Figure 2]

The modulation profile of the refractive index of the grating used to form the outer cavity is shown.

[Fig. 3]

It is a graph which can evaluate the performance of the laser by this invention.

[Fig. 4]

It is a graph which can evaluate the performance of the laser by this invention.

[Fig. 5]

It is a graph which can evaluate the performance of the laser by this invention.

[Explanation of symbols]

1 hybrid laser 2 Fabry-Perot cavity active element 3 facet 4 optical fiber 5 grating 6 Tapered end 7 Support stand 8 resin