Optical equalization receiver for lightwave communication systems

2~2~309 OPI ICAL EQUAI,IZATION RECEIVER FOR LIGHTWAVE COMMUNICATION SYSTEMS Technical Field This invention relates to the field of lightwave systems for dispersion 5 equ~li7~tion in which the system compensates for the effects of dispersion within a tr~n~micsion medium.

- Bach~l ou..d of the Invention The effects of delay distortion, specifiç~lly those associated with fiber chloll.alic dispersion, have limited bit rates and repeater spans of convention~l 10 lightwave co,n..,-~ic~tion systems.

The underlying mechanism that causes thislimit~tion is intersymbol interference.

In ~cl(lition, direct mod~ tion of singlelongihldin~l mode semiconductor lasers causes a transient wavelength chirping (laser chirp), which in comhin~tion with the chromatic dispersion of the fiber, causes increased intersymbol interference.

Laser chirp alone has no ~elçt~rious effect on 15 direct detection optical links unless there is dispersion in the tr~n~mi~cion m~lillm, while fiber dispersion alone, with no laser chirp present, can still limit the bit rate and repeater span resulting from associated intersymbol in~lr~;llce.

The effects of laser chirp combined with fiber dispersion are discussed in articles by P. J.

Corvini et al., J.

Li~htwave Tech., Vol. LT-5, No. 11, pp. 1591-5 (1987), D. A.

Atlas et al., 20 Optics Lett., Vol. 13, No. 11, pp. 1035-7 (1988), and J. C.

Cartledge et al., J.

Lightwave Tech., Vol. 7, No. 3, pp. 568-73 (1989).

Further, the effects of fiberdispersion alone are ~liscusse~, for example, in the article by A. F.

Elrefaie et al., J.

Lightwave Tech., Vol. 6, No. 5, pp. 704-6 (1988).

With the trend toward higher bit rates and longer repeater spans, various 25 equ~li7~tion techniques have been proposed to reduce the effects of laser chirp.

Approaches include use of optical pulse shaping and external mod~ tion techniques.

See, D.

Le Clerc et al, Proceedin~s f 1985 IOOC-ECOC, pp. 775-8.

Additionally, techniques employing semiconductor laser amplifiers have been used to impose a chirp on a tr~n~mitted optical signal to counteract the combined effects of fiber 30 dispersion and laser chirp.

See, N. A.

Olsson et al., Electron.

Lett., Vol. 25, No. 9, pp. 603-5 (1989).

, 2026309 - 2 The above approaches attempt to reduce the effects of laser chirp directly at the tr~n~mitter source, i.e., the laser.

An ~ltern~tive approach focuses on reducing the reslllting effects of dispersion directly at the receiver by utili~ing a tr~nsmi~cion medium having a strong chromatic dispersion opposite to that of the5 tr~n~mic~ion fiber.

Particularly, single-mode fibers, dispersive grating and optical tapped delay lines have been demonstrated to reduce the effects of laser chirp and fiber dispersion.

See, D.

Marcuse, Appl.

Optics, Vol. 20, No. 4, pp. 696-9 (1981) and L. B.

Jeunhomme, Single Mode Fiber Optics, ~ 213-5 (lg83).

In all of the approaches above relating to eqll~li7~tinn~ there exist 10 problems in achieving low optical power penalty while m~int~ining low cost and simplicity.

Summary of the Invention An optical eql-~li7~tion receiver for countering the effects of delay distortion, specifically fiber chromatic dispersion, is realized by utilizing 15 dyn~mir~lly controllable Fabry-Perot etalon structures.

By dyn~mic~lly adjusting the frequency response of an etalon that is in a feedb~ck loop configuration, the associated time delay characteristic of the etalon may be used to counter the effects of delay distortion relating to the fiber chromatic dispersion, thereby allowing higher tr~n~mi~ion bit rates or longer tr~nsmi~ion distances to be achieved.

In one exemplary embodiment, an optical receiver comprising a reflective Fabry-Perot etalon and a piezoelectric transducer for dyn~mic~lly controlling the optical path length of the etalon are used in a feedback loop configuration wherein a feedback circuit monitors an output optical signal from an end of the etalon for generating a control signal representing the amount of delay 25 needed to compellsate for distortion in an optical signal incident on the etalon.

Specific~lly, the control signal causes a change in the optical path length of the etalon that results in a shift of the frequency response of the etalon for producing a delay characteristic substantially equal and opposite to the associated delay distortion of the tr~n~mi~ion medium.

Various feedback control strategies may be used in adjusting the frequency response of the etalon so as to obtain a delay characteristic substantially equal and opposite delay to the associated delay distortion.

For instance, in returnto-zero signaling, the etalon frequency response may be adjusted in real time so as to maximize the sinusoidal amplitude component of the detected optical signal at the 35 frequency of the ~ign~ling bit-rate.

Further, for both return-to-zero and non-return ~0263as to-zero sign~lin~, another feedback control strategy is to adjust the frequency response of the etalon such that the "eye opening" of the detected optical signal is m~ximi7ed In accordance with the principles of the invention, the frequency S response of the etalon may be adjusted in accordance with the above feedb~c~ strategies by changing the optical path length of the etalon, such as by varying the cavity length of the etalon, or by inducing a change in the refractive index of the optical me lillm enclosed within the cavity of the etalon.

Brief Description of the Drawin~ The invention will be more readily understood after reading the following de~iled description of specific illustrative embo~liment.s of the invention in conjunction with the appended drawings wherein:

FIG. 1 shows an exemplary lightwave co".,-~",ication system for illustrating the principles of the invention;

FIG. 2 shows an exemplary embodiment of a reflective Fabry-Perot etalon structure;

FIG. 3 shows the norm~li7ed delay response of the etalon structure shown in FIG. 2;

FIG. 4 shows an exemplary embodiment of a tr~n~mi~ive Fabry-Perot 20 etalon structure;

FIGs. 5-6 show the amplitude and delay characteristics of the tr~n~micsive etalon structure shown in FM. 4;

FIGs. 7-9 show the optical power penalty versus h~n~mi~sion distance forvarious etalon structures as compared to an unequalized cc,l"l"~ ication system;

25 and FIGs. 10 through 12 show the optical power penalty versus tr~nsmi~sion distance for dirrt;~.lt fee~lb~ control strategies.

Detailed Description A lightwave receiver that compensates for delay distortion in lightwave 30 co~ unication systems is reali~ed by employing a dyn~mic~lly controllable FabryPerot etalon structure.

The present invention is based upon the discovery that Fabry-Perot etalons may be used to counteract the effects of fiber dispersion, which effects can be particularly clelçt~ric)us for optical signals with laser chirp.

Specifically, fiber is observed to have a frequency response constant in amplitude and linear in time delay, with a slope polarity that depends on wavelength.

On the other hand, the frequency response of a Fabry-Perot etalon similarly has, over alimited range of frequencies, a substantially linear time delay with either a positive slope or negative slope.

By c~sc~ling the dyn~mi~ally controllable etalon structure 5 with a fiber and p~elly positioning the etalon frequency response via a feedback control loop, a delay characteristic complementary to that of the fiber can be generated for effectively reducing the effects of fiber dispersion over a certain range of frequencies, i.e., wavelengths.

Shown in FIG. 1 is an exemplary block diagram of a lightwave 10 co....~ tion system utili7ing the present lightwave receiver for optical equ~li7~tion.

It is to be understood, however, that the co.. nic~tion system depicted in FIG. 1 is for the purpose of illustration only and not for the purpose of limit~tion As shown in FIG. 1, optical source 101 is a semiconductor laser operating in a single-longitll-lin~l mode which is directly modulated by a current, I(t) 15 of the form:

00 I(t) = Ibias+ ~ akIp(t - kTb) (1) k=~ where ak is a data sequence of "0" 's and "1" 's, Ip(t) is the tr~n~mitted current pulse and 1/Tb is the signaling bit-rate.

Any ~ign~ling format, such as return-to-zero (RZ) and non-return-to-zero (NRZ), can be used for tr~n~mitting optical signal 100 over a 20 tr~n~mi~sion merlillm Current waveform, I(t) is shown to be filtered by pre-source filter 102 to account for laser parasitics as well as for pulse sh~ping of optical signal 100.

Pre-source filter 102 may be, for example, a simple RC circuit or the like.

The response of optical source 101 to the filtered current waveform is det~rmined by solving the large-signal rate equations, which describe the 25 interrel~ion~hir among photon density, carrier density and optical phase within the laser cavity.

The reslllting electrical field of the laser output, in any case, can be represented as E(t) = ~ej~(t) (2) where P(t) is the laser facet power and ~(t) is the laser phase.

Furthermore, a simple 30 relationship exists between the wavelength excursion and the time dependence of the optical power output from the laser.

The laser chirp, which is the time derivative of 2~26309 the laser phase, is approximately given by:

d~ a ~ d lnP(t)+KP(t) ~ (3) where a is the linewidth enh~n~ement factor and lc is a ~ et~l dependent on the physical structure of the laser.

For long-haul tr~n~mi~sion, the laser output, E(t), is tr~n~mittefl over single-mode fiber 104 which is L km in length.

Modeling fiber 104 as a b~n~r~s filter with a flat amplitude response and a linear group delay, the frequency response is given by:

L~,2D~2 Hf(C~)=e 47~c (4) 10 where D is the fiber dispersion, ~ is the wavelen~th, c is the speed of light in free space, and ~3 is radian frequency measured from the center frequency of optical source 101.

The flat loss of the fiber is irrelevant to this discussion and thus omitted.

It should be noted that the slope of the group delay is deterrnin~d by the chromatic dispersion of fiber 104.

Not incl~1ded in the above description are attenuation and 15 higher-order terms in the phase response.

The optical output of fiber 104 is affected by Fabry-Perot etalon structure 105 before passing to optical detector 106, which collvel ls the ~ietected optical signal into an associated electrical signal.

It should be noted that optical receiver 112 comprises dynamically controllable etalon structure lOS, opdcal detector 106 and feedback control circuit 107.

Feedback 20 control circuit 107 affords a means for adapting the frequency response, H~ of etalon structure 105 by m~ uring a predetermined electrical characteristic of electrical signal 109 and adjusting the etalon frequency response according to apre~letermined feedbflc~ control strategy so that etalon structure 105 operates at a point on its delay characterisdc for producing delay substandally equal and opposite 25 to the associated delay distordon of fiber 104.

Ful~ ore, the electric field output of etalon structure 105 is squared in m~pnitll-le by opdcal detector 106 and passed through post-detector filter 108, which may be omitted.

Post-~etector filter 108 may be, for example, an nth order But~l V,/Ol lh low-pass filter.

,~ 2~3~9 - 6 The source-to-detector section consists simply of fiber 104 and etalon structure 105.

The impulse response, hf(t), of fiber 104 alone is of the form ei~t where ,~ is ~c/~2DL.

Those skilled in the art will recognize that the phase response is derivable from equation (4) above.

In accordance with equation (4), the fiber5 impulse response, hf(t), has a constant amplitude with tdme and a phase variation corresponding to a linear chirp, i.e., quadratic phase response.

That is, the phase has a linear frequency variation given by: v(t) = ~ )t Hz.

The averall impulse response hSyS(t) of fiber 104 and etalon structure 105 is desired to have the form of a delta functdon, i.e., "impulsive".

This is 10 because if fiber 104 in combinadon with etalon structure 105 has an impulse response of the form o (t), the signal, E(t), propagates unaltered through fiber 104 and etalon structure 105.

It is noted that the overall impulse response, hSyS(t) is given by heq(t) * hf(t) or, in the frequency domain, the overall frequency response, Hsys (co) is given by Heq(c~) x Hf(~).

In this particular case, two necessary conditions for appro~cim~ting an "impulsive" response exist.

An additional condition is required if there exists any laser chirp resuldng from directly modulating opdcal source 101.

First, there should be one peak of hSyS(t) for which the associated fiber chirp frequency is a~lv~i,llately zero.

This condition is s~ticfied by having the first or plilll~y peak of the frequency 20 response close to the time origin and, moreover, is achieved through the proper choice of the frequency shift of Heq(cl~).

For example, a simple way to position(frequency shift) the frequency response of etalon structure 105 is to pie7Oelec~ic~1ly change the cavity length of an etalon within the structure by apiezoelectric tr~ncdllcP.r.

The change in cavity length required to move the response 25 a full period is half of an optical wavelength.

Second, the nominal width of the frequency response's ~li"laly peak should be appl. kimately less than a bit period.

This is achieved through a judicious selection of the mirrors' reflectivity for a given cavity length of the etalon structure.

Third, ~ccllming that laser chirp is present, the chirp frequency associated with each of the other peaks of hSyS (t) should be large 30 col~a~ed to the largest chirp frequency in the laser pulses.

This condition ensures that the convolution of secondary impulse response peaks with the input signal is negligible and, moreover, is achieved by choosing the rr~uellcy response period of the etalon structure to be sl1bst~n~ ly greater than the highest chirp frequency.

In one exemplary embodiment, a reflective single-cavity Fabry-Perot 35 etalon and a pie7Oelectric tr~ncd~lcer may be used as dyn~mi~lly controllable etalon structure 105.

As shown in FIG. 2, back rr~i~ror 202 having a reflectively close to ~2630g 100% and front mirror 201 having a power reflectivity of r2 in combination with optical m~ m 203 (refractive index - 1.5) are arranged to form reflective etalonstructure 200.

Additionally, a piezoelectric transducer (not shown) may be used as the means for dyn~mic~lly varying the cavity length of etalon 200.

Output optical 5 signal 110 from fiber 104 is coupled optically by lenses (not shown) into fiber 205.

Reflective etalon 200 reflects optical signal 110 and, moreover, introduces an associated delay into optical signal 110 to generate optical signal 111.

Three dB coupler 207 couples optical signal 111 into fiber 206, which directs optical signal 111 to optical ~letector 106.

The reflective etalon and piezoelectric tran~duce 10 may be implçmen~ed as described in U.S.

Patent 4,830,451.

This reference cited above and its teachings are expressly incorporated herein by reference.

In addition, an optical circulator, instead of 3 dB coupler 205, may be used to couple in and out of reflective etalon 200.

See, for example, I.

Tokc h~m~ et al., Electron Lett., Vol.

22, No. 7 pp. 370-2 (1986).

The 3 dB coupler implementation although structurally 15 simpler than the circulator design by Tokohama et al. has a higher loss.

Other implementations for reflective etalon structures, such as those comprising more than one reflective etalon, may be used instead of a single cavity reflective etalon structure, see R.C.

Alferness et al., Electron Lett., Vol. 24, No. 3, pp. 150-1 (1988) In all these reflective etalons, however, the frequency response, Heq(cl)), 20 is given by:

H (~) A < 1 + re)~T = Ae~ ~( ) 1 +rcos~T (6) where T is the round-trip delay time of the cavity, and A is a constant representing 25 the loss of the structure.

As shown in FM. 3, the norrn~li7~1 delay response,~(~)/T, is found to be periodic in frequency with a period of l/T Hz and, moreover, from the above equation is given by:

2~3~9 d~r(c )) d~ ~ l+r2~2rcosct)T, It is contem~ te~l that the linear portion of the delay response would be used to counter the delay dispersion of fiber 104.

It should be noted that the delay response 5 of etalon 200 should have the ap~lo~-iate polarity and m~tgnitude to counteract the delay response of the fiber.

When RZ signaling is used for the co-.""ll,-ic~tion system shown in FIG. 1, electrical signal 109 contains a sinusoidal c~ nlpolle.lt at the signaling bit-rate l/Tb of optical signal 100.

One exemplary method of adaptively controlling the 10 frequency response of etalon 200 is to position the frequency response so as to m~Ximi7e the amplitude of this sinusoidal co,~ollent.

Such a method approximately co~ ollds to the narrowest, best-equalized output electrical signal 109.

This can be achieved by feedback control circuit 107 periodically m~em ing the amplitude of the sinusoidal component of electrical signal 109 and 15 using the relative change from previous mea~ulemcnts as a control signal for ch~nging the cavity length of etalon 200 via the piezoelectric tr~n~d~lcer.

While ma7cimi7ing the amplitude of the frequency component of electrical signal 109 at the .~ign~ling frequency, l/Tb, may be used as a fee-lh~ck control strategy, it cannot be used in the case of NRZ sign~ling This is because for ~RZ .si n~ling, electrical20 signal 109 contains no frequency component at the ~ign~ling bit-rate.

However, other fee~lb~ck control strategies may be used.

For NRZ ~ign~ling, the "eye opening" versus frequency profile has a convex shape.

Thus, one method of adaptively controlling the frequency response of etalon 200 is to position the frequency response of the etalon so as to maximize the "eye opening" of electrical 25 signal 109.

In another exemplary embodiment, instead of using reflective FabryPerot etalon structures, tr~n~mi.~sive etalon structures may be used that utilize a simple, single cavity tr~n.~mi.~.cive etalon, shown in FIG. 4.

For tr~n~mi.~ive etalon 405, the frequency response is given by:

6 3 0 ~ .

g l_r2 ~(~) (8) where the phase response is:

tan-l r2sinc~)T ~ l-r2cos~T, and the colles~onding delay response is S ~ ) = r2T~ COs~3T-r2 (10) ~ l+r4-2r2coscl)T, Similar to the previous reflective etalon, r2 is the power reflectivity of mirrors 401, T is the round-trip delay of the cavity and the free spectral range (FSR) is simply given by 1/T.

It should also be noted that optical m~illm 402 is enclosed within mirrors 401.

Illustrative amplitude and delay char~cteristics for this Fabry-Perot 10 etalon are shown in FIGs. 5-6.

For tr~n~missive Fabry-Perot etalon structures, a simple feedback control strategy may be employed and that is to center the optical signal spectrum of optical signal 110 at the peak of the etalon tr~n~mi~cion response.

This can be accomplishe(l by maximizing the dc power detected in electrical signal 109.

15 Furthermore, this feeclb~çk control strategy works for both NRZ as well as RZ ~ign~ling.

In an example from computer cimnl~tinn, optical signal 100 (signaling bit-rate ~ 8 Gbs/sec) was modeled by nllmeric~lly integrating the large-signal rate equations using the following parameters in which optical source 101 was modeled20 as a distributed feedb~c~ buried heterostructure laser: ~1.5,um; W=1.5~m; rv=0-4;

Nt-lxlOl6cm~3; ~ph=1.4ps; ~e=l.Ons; ,Bsp=3.9xlO~cm/s; vg=7.9xlO9cm/s;

C=lO.Ocm2/s; a=2.5xlO-6cm3/s; =l.SxlO~l7cm~3; a~6; Pm",=8.3mW; and Pmin=1.7mW.

Here, ~ is the wavelength, W is the width of the active layer, rv is the mode confinement factor in the vertical direction, Nt is the carrier density required 25 for transparency, 'Cph iS the photon lifetime, 'Ce is the carrier lifetime"~5p is the fraction of spontaneous emission into the lasing mode, l)g is the group velocity, C is the carrier diffusion coefficient, a is the gain coefficient, iS the gain com~lession coefficient, Pma~C is the maximum power output; and Pmin is the minimllm power 2~6~g - 10output.

Pre-source filter 102 in the laser driver circuit had a 3 dB bandwidth of 4 GHz, while post-detector filter 109 had a bandwidth of 6.24 GHz, corresponding to 0.78/Tb where Tb is the signaling period.

In addition, fiber 104 had a dispersion, D, 5 of - 17 ps/nm/km.

Optical signal 100 was generated using a maximal-length pseudor~nclom sequence of length 64 which contained all bit sequences of length 6.

Shown in FIGs. 7, 8 and 9 are the perfnrm~nce results.

It should be noted that these results demonstrate typical improvements that may be obtained by ~ltili7ing the present optical equalization receiver.

The results show optical signal 10 power penalty versus tr~n~mission distance for both NRZ and RZ si~n~ling.

In both cases, the optical penalty for an unequalized system is co.ll~aled to those systems utilizing both reflective and tr~n~mi~sive Fabry-Perot etalon structures.

The results in FIGs. 7 and 8 include the effects of laser chirp, while FIG. 9 does not.

Reflectivity, r, and cavity delay, T, were chosen based on conditions two 15 and three stated hereinabove for obtaining an "impulsive" response.

Furthermore, for achieving condition one, the etalon phase response has been positioned by searching over an entire period of the delay characteristic of electrical signal 109 to find that position yielding the ma~ m "eye opening".

The positioning of the phase response results in reducing the fiber delay distortion.

In general, sust~ining a 20 specified linear phase response over a specified signal bandwidth is not possible regardless of the values of r and T.

Nevertheless, the eqn~li7~tion receiver can still provide increased system performance as measured by the optical signal power penalty.

Here, the optical penalty is defined as the "eye opening" relative to the case of a fiber having a length zero and no post-detector filter present.

The optical penalty 25 measured at the output of post-detector ilter 108 includes cl~u,llatic dispersion of the fiber, laser bandwidth limit~tions and nonlinearities, and receiver bandwidth limit~ti--ns.

Ln the case of NRZ ~ign~ling, the following etalon structures were employed: a reflective etalon having a reflectivity of r= 0.82 and a round-trip delay 30 time of T= 12.5 ps; and a tr~n~mi~sive etalon having a reflectivity of r=0.86 and a round trip delay of T= 12.5 ps.

For the tr~nsmi~sive etalon, the p~r~mçters correspond to a free spectral range of 80 GHz and a 3 dB bandwidth of 7.6 GHz.

Additionally, the choice of r and T are near-optimal.

Particularly, r and T werechosen to minimi7e the optical power penalty for a specific length of fiber, namely, 35 I,=100 km.

The use of eq~li7~tion significantly extencled the range of "eye-open" operation.

For example, FIG. 7 shows that optical e~ li7~tion can m~int~in an ~a2~ - 11"open eye" at distances more than twice those distances at which the unequalizedsystem becomes inoperable.

For RZ signaling (duty cycle ~ 0.75), similar resultswere obtained, see FIG. 8.

In this case, the reflective etalon structure had thepala,l.e~ r=0.78 and T=12.5 ps while the tr~n~mi~sive etalon structure had the S p~dlllelel . r=0.82 and T=12.5 ps.

Although, the improvements are not as substandal as for NRZ sign~ling, the "eye opening" still remains open for a distance much greater tnat 100 km.

The results for optical power penalty shown in FIGs. 7-8 do not account for structural losses, which range from - 2 dB to 6 dB.

These losses, along with the 10 fiber ~ttenll~tiQn, must be taken into con~i~ler~tion in ~letermining the permissible tr~n~mi~ion distance.

Figure 9 shows that in a commnnication system without laser chirp, utilizing a receiver compri~ing a controllable reflective etalon ~ c~ule, the effects of dispersion alone can also be substantially reduced, increasing the achievable 15 tr~n~mis~ion distance by at least a factor of two.

Figures 10 through 12 show the performance in terms of optical power penalty versus tr~n~mi~ion distance for different control feedback strategies used in positioning the frequency response of the etalon structure.

It should be recalled that for the reflective etalon structure and the case of RZ sign~ling, one possible feedback 20 control strategy is to m~ximi7e the dc power of electrical signal 109 at the bit-rate frequency, i.e., maximize the first harmonic peak.

However, another control strategy is to m~ximi7e the "eye opening" of electrical signal 109.

Either control strategy is accomrlished by adjusting the cavity length of the etalon via the piezoelectric tr~n~tlucer by feedbacl~ control circuit 107, which is well known in the prior art.

25 Feedback control circuit 107 may utilize differenti~rc, decision circuits, band-pass filters, integrate and dump circuits, envelope detectors and the like for measuring the "eye opening", dc power, or an sinusoidal amplitude cc,lllpollent of electrical signal 109.

Figure 10 indicates that ma~imi7ing the first harmonic peak works reasonably well for distances from 40 km to about 120 km.

For ~n~mi~ive etalon 30 structures, the etalon response is centered on the center optical frequency of optical signal 110, i.e., at the Fabry-Perot tran~mi~sic)n peak.

As indicated by F~Gs. 11 and 12, this fee~lb~c~ control strategy works well for both RZ and NRZ ~ign~ling.

~ dditional results show that the ~e.ro~ ance of the lightwave receiver is not very sensitive to variations in the reflectivity or the round trip delay time of the 35 etalon.

For example, the optical signal power penalty varies only O.S dB for round trip delays between 10 ps and 25 ps.

This assumes a fiber length of 100 km, and a - 12- ~Q2S3~9 reflective etalon structure with reflectivity, r, of 0.74.

Also, the optical signal power penalty varies less than 0.2 dB for reflectivities between 0.74 and 0.82 for a fixed round trip delay of 12.5 ps.

With con.~i~e7~fion to the bandwidth of post--7.etector filter 108, it was S found that the pe rolmallce of both unequalized and equalized tr~n~m~ f)n systems was sensitive to the type of post-detector filter and, moreover, its bandwidth.

In the results presente-7, post-detector filter 108 had a bandwidth of 6.24 GHz.

For RZ~i~n~ling, further computer simnl,7tion inrlic~tec7, that increasing the filter bandwidth to 7.55 GHz (r re-optimized to 0.74) reduced the optical power penalty from 3.0 dB 10 to 2.26 dB.

Also, different laser parameters could alter the optical power penalty for the given system paldm~Lel~.

While a single sign~ling bit-rate, B, was used for the data generated, it is anticipated that the product B2-L will be approximately constant for a given optical power penalty.

Referring to FIGs. 7 and 8 when some method of optical 15 eqll~7li7~tion was employed, the product B2-L increased, for NRZ ~igJ7~71ing, from 6400 (Gb/s)2 km to better than 25600 (Gb/s)2 km and, for RZ .~i n~ling, from 5760 (Gb/sec)2 km to better than 10240 (Gb/sec)2 km.

Thus, for example, at a sign~ling bit-rate of 16 Gb/s, the unequalized system will be inoperable above 25 km, while an equalized system should m~int~in an "open eye" pattern for fiber 20 lengths greater than 50 krn.

With respect to timing recovery and threshold considerations, the midpoint of the "eye opening" may be attained as the threshold setting since it corresponds a~pr~ ately to the average dc power level at optical c7,~.tectc r 106.

Furthermore, for the case of RZ ~ign~ling, the frequency component at the sign~ling 25 frequency of electrical signal 109 may be used to derive the sampling phase for optical ,7etector 106.

Various other mo~iifi~til)ns may be made by those skilled in the art which will embody the principles of the invention and fall within the spirit and the scope thereof.

For instance, multiple cavity etalon structures may be used instead of 30 a single cavity structure.

See, for example, A. A. M.

Saleh et a7.., Journal of Li~htwave Technolo~y, Vol. 7, No. 2 pp. 323-30 (1989).

Theoretically, these multiple structures should improve the system's ~ rc,~ ance.

Also, the frequencyresponse of the various etalon st~uctures may be adjusted in accordance with theabove fee lb~c~ strategies by inducing a change in the refractive index of the optical 35 m~inm enclosed within the etalon cavity.