4:30pm - 5:O-m (Invited) ME3 - _-

Optical Switching in InGaAsP Amplifiers

D.A.O. Davies B.T. Laboratories

Martlesham Heath, Ipswich, Suffolk, U.K.

Introduction

Future optical communications networks will require large bandwidth, high capacity switching systems that can be controlled as easily and flexibly as possible; in a variety of applications involving high bit rate systems, photonic switching appears to offer advantages in the provision of space, time and wavelength switching. Optical amplifiers based on InGaAsP material technology are proving to be very flexible when employed in such switching devices. As well as offering compact, readily controlled gates with a considerable ONIOFF ratio, their inherent gain can overcome splitting losses in spatial switching arrays (l), while their nonlinear and spectral properties allow a range of functions to be implemented suitable for high speed and multi-wavelength systems. Means of exploiting these properties can roughly be divided into devices where the optical path is controlled electronically via the injection current, and those with a d.c. injection current where incoming intense optical pulses cause modulation via nonlinear effects in the device active region. Examples of both will be discussed in this paper.

Electronically Controlled Switching

The modulation of the injection current into a semiconductor laser amplifier leads directly to the modulation of the optical gain and refractive index of the active region. The gain modulation, which can have an extinction ratio of > 40 dB, can be used to provide highly effective optical gating, while the optical gain can be used to provide zero insertion loss (2).

One way to extend the functionality of a simple gating device is to incorporate it into a directional switch with several ports. This has been done in the twin-guide amplifier (TGA) (3), where an active directional coupler is fabricated using a modification of laser ridge waveguide technology (see figure 1)

i2

Figure I : twin guide amplifier: currents il and i2 pump the waveguides separately. Typically the waveguides are

2 pm wide, with a device length of - 500 pn.

This device is constructed so that for equal currents into the two ridges the switch is in a cross state (i.e. light

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entering guide 1 leaves from guide 2). For unbalanced injection currents, the combined effects of the real and imaginary refractive index changes in the device active region cause the device to switch to a bar state (light leaves from the same waveguide that it is injected into). This device can be lossless fibre-to-fibre, and can operate with a crosstalk of better than -33 dB for an injection current of 140 mA.

The TGA thus provides a building block for a larger optical space switch, where several devices are interconnected. Although this could be done in a hybrid system with fibre connections, a far more elegant solution is to integrate several devices to form a larger functional unit. We have demonstrated this concept for the case of a 1 to 4 optical space switch (4) by integrating together three twin guide devices. The device layout is shown in figure 2. The waveguides are all 3 pm wide ridges. The twin-guide coupler regions are 370 pm long, with a guide separation of 2 pm, and are interconnected by curved waveguides of radius 2 mm. The overall length of the device is 2.8 mm. This is partially determined by the need to separate the output waveguides to a pitch of 128 pm to allow access by a fibre array. The device consists completely of active waveguides pumped through the various current contacts indicated in figure 2. In this simple configuration, the twin-guide coupler regions act as power splitters, with a common current contact to the two ridges. Switching is then possible between port X and one of the ports A, B, C, and D. Optical transmission is possible in either direction, allowing use as a splitter or combiner.

An optical "connection" is established between input and output ports by electrically pumping all the intervening regions. Clearly many possible combinations of currents are possible, but a simple method is to inject constant current into regions 5 , 6, 8, 9 and 12, while using the

c---------)

4 * 2.8 mm

370 pm

Figure 2 I x 4 .witch. Switching is between port X and ports A, B, C and D. The waveguide ports above and below X are for testing only. The numbered shaded regions are isolated current injection contacts.

currents to regions 1 to 4 to perform the channel select function.

Lossless operation fibre-to-fibre can he obtained in this type of device for TE polarised light using control currents of less than 100 mA. The 3 dB bandwidth of the gain has been found to be 20 nm. However, since the coupling length of the twin-guide sections is polarisation dependent, the performance for TM light was considerably degraded, and gain measurements could not be made for all the paths.

These results demonstrate the possibility of producing lossless integrated devices based on 1.55 pm semiconductor laser amplificr technology. However, properties such as crosstalk, bandwidth, polarisation sensitivity, noise and scaleability will also be important in determining the performance of a device in a switching application. In this device, particular issues to be examined include the noise and power consumption characteristics of a device using active waveguides throughout, as well as the polarisation sensitivity of the directional coupler structure. Further, the use of curved waveguides and dircctional couplers mean that the device size will become impractically large if more complex switching functions involving additional channels are to he produced.

An alternative approach that has more recently been developed may allow the production of larger switch arrays. A 2 x 2 switch based on this technology is shown in figure 3. This involves the use of total internal reflection mirrors to produce waveguide comers as well as to produce integrated beam splitters (5 ) . The waveguides are all passive with the exception of the amplifier sections. A path is simply selected in this device by driving the appropriate amplifier. Recent results show lossless fibre-to-fibre operation for both TE and TM polarisations with an average polarisation sensitivity of 4 dB (6). With the use of integrated passive and active

waveguides, this type of device offers a technological route to the development of compact components with a reduced current consumption and which are capable of being scaled up to produce larger switching arrays. Consequently, one can see that semiconductor laser amplifiers controlled by modulation of their injection current can be effective building blocks for the production of switching devices which will impact on future optical fibre communication systems.

Input

Passive waveguide Amplifiers Output

Figure 3: device structure for a 2 x 2 switch employing reflective waveguide angles and power splitters

Optically Controlled Switching

In addition to the electronically controlled space switching devices described above, there is considerable interest in investigating devices where the control signal is optical, enabling one light beam to control another. This type of device may be attractive for a number of reasons: some functions may be performed more easily by removing the need to implement optical-electrical-optical conversions, some functions (espccially those using wavelength) exploit properties of light and are best suited to manipulation in the optical domain, and there may also be speed advantages, hoth because optics can exploit some

ultra-high speed effects, and also because of the larger bandwidth of optical interconnections. In this nonlinear regime IdJ based semiconductor laser amplifiers can also find application.

There are a number of ways in which optical nonlinearities can be exploited to produce time, space and wavelength switching of optical data. All require the modification of the optical properties of the device material by a switching beam, which is then sampled by a second distinct beam. The easiest way to do this in an amplifier is via gain saturation. At high light levels the rate of carrier injection into the device active region is insufficient to replenish the carriers being removed by stimulated emission. The carrier concentration in the active region then falls, reducing the optical gain and increasing the refractive index. Both properties can be used to produce a switch, either simply based on the change in transmission or using the refractive index change in an interferometric configuration. The nonlinear effects in this mode of operation are large, and are assisted by the amplifier gain. For pulses shorter than the carrier lifetime, only picoJoule energies are required to give enough phase shift to produce a switch.

The rate of recovery of the perturbation to the carrier number is limited by the carrier lifetime in the device active region (typically 500 ps to 1 ns), limiting the bandwidth of the response to a few GHz. However, recent work has shown that gain saturation effect can be used to produce switching at considerably higher bandwidths, either by increasing the optical power in the active region (7) (20 Gbit/s wavelength conversion), or by using the nonlinearity to perform demultiplexing, with a regular periodic switching beam (8) (10 Gbit/s demultiplexed from 40 Gbit/s ).

However, for operation with a data bearing switching beam at bit rates high enough to compete with future electronic performance, it seems that these nonlinear effects may be too slow. In this case, one can look to higher speed nonlinear effects within semiconductor laser amplifiers. In particular, recent results using self-phase modulation (9) and pump-probe techniques (10) have shown the presence of gain and refractive index nonlinearities in semiconductor laser amplifiers that recover with sub-picosecond time constants. These effects are attributed to the combined action of carrier heating and spectral hole burning. They are also responsible for recent results on four-wave mixing in amplifiers at large detunings between the pump and probe (1 1). For devices where these effects are to be used to switch high-speed data, the lower speed effects associated with longer term changes in carrier population changes need to be avoided. In four-wave mixing this is achieved by the C.W. pump beam holding the device gain clamped. Degenerate four- wave mixing has recently been used to demonstrate wavelength conversion of data signals of over 20 nm (12). Another method of using the high-speed effects to achieve optical switching is suggested by the results in references

9 and 10, and is based on biasing the device at transparency (material gain is zero). In this regime the net transition rate induced by an incoming optical beam from conduction band to valence band balances the rate from valence band to conduction band. Viewed simply, one can see that an incoming beam should not affect the carrier numbers in the two bands, removing any problems associated with long term changes in carrier number.

Self-phase modulation experiments performed with an amplifier biased at transparency show that the nonlinear effect in this regime is large, requiring 1 to 2 W peak power in a 15 ps pulse to give a peak phase shift of n (sufficient to switch an interferometric device), and that the recovery time is of the order of a picosecond. We have attempted to exploit this nonlinear effect in an integrated nonlinear directional coupler (13). The device is similar to that shown in figure 1, except that the current contact was common to the two waveguides. When biased in the transparent regime, the nonlinear switching characteristics shown in figure 4 were observed.

j ;:;;- transmission

0 2 4 6 8 1 0 1 2 Average power in (mW)

Figure 4: nonlinear directional coupler response biased at transparency. Optical pulses of duration 15 ps were

used: an average power of I O m W corresponds to a peak power of 8.9W.

As can be seen, at low powers nearly all the input light was coupled into the cross output port, but that at increasing input powers the proportion of light emerging from the bar port rose to about 40%. This demonstrates the possibility of switching using this approach, but the poor switching ratio achieved illustrates the detrimental effect of internal loss on devices operating at transparency; although the amplifier is transparent in terms of band to band transitions, the internal loss caused by effects such as scattering and free carrier absorption still remains. Of the several possible interferometric devices available to exploit this effect, the nonlinear directional coupler response is particularly sensitive to the effects of loss. As recent experiments have shown (14), quantum well devices may offer the hope of reduced internal loss while maintaining the size of the nonlinear effect.

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Conclusions

For light in the telecommunications windows of 1.3 pm and 1.55 pm, devices based on InP / InGaAsP semiconductor laser amplifiers offer a range of properties suited to providing optical switching. Electrically controlled devices can offer switching that is essentially transparent to optical data, with the possibility of producing complex integrated components. Optically controlled devices can benefit from the range of nonlinear effects available in laser amplifiers, with a trade-off being available between slower but large gain saturation effects, and smaller but very fast intra-band effects.

Acknowledgements

The results presented here are due to the following colleagues at B.T. Laboratories: M.J. Adams, J.D. Burton, D.J. Elton, P.J. Fiddyment, M.A. Fisher, A.E. Kelly, D.A.H. Mace, P.S. Mudhar, S.D. Pemn, M.J. Robertson, C.P. Seltzer, G. Sherlock, J. Singh, and P.C. Sully, as well as R.S. Grant, G.T. Kennedy, P.D. Roberts and W. Sibbett at the Dept. of Physics and Astronomy, St. Andrews University, U.K.

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