GaAIAs SEMICONDUCTOR DIODE LASER 4-ARY FREQUENCY SHIFT KEY MODULATION AT 100 Mbit/s Indexing terms: Semiconductor devices and materials, Lasers and laser applications 4-ary FSK modulation of a semiconductor diode laser by injection current modulation has been demonstrated at a rate of 100 Mbit/s. The injection current modulator has a modular design which allows expansion to M-ary FSK with independent control of each tone frequency. The design allows considerable flexibility in the logical relationship between the source data and the selected frequency tones. Introduction: Semiconductor diode laser based optical com- munications systems generally use intensity modulation with direct detection. However, the advent of commercially avail- able single-frequency diode lasers has generated renewed interest in frequency and/or phase modulation with hetero- dyne detection. These schemes promise more than 10 dB in performance gains over the present direct detection tech- niques. 1 Frequency modulation of a diode laser can be obtained by injection current modulation, 2 whereas phase modulation involves the use of an external phase modulator or the optically complex process of injection locking. 3 This letter reports, for the first time, 4-ary frequency shift key modulation of a semiconductor diode laser at a rate of 100 Mbit/s. The received power levels in present diode laser systems generally preclude the use of coherent detection with a phase tracking receiver. 4 Noncoherent detection with frequency tracking has been demonstrated using current technology. 5 This limits the optimum modulation scheme, for an ideal zero linewidth laser, to orthogonal M-ary FSK. 6 Orthogonal sig- nalling at 50 M symbols/s (since each symbol transmits 2 bits the data rate is 100 Mbit/s) requires tone spacings that are multiples of 50 MHz. However, the nonzero linewidth of the laser destroys complete orthogonality with the minimum crosstalk now occurring for tones spaced by multiples of 50 MHz plus the laser linewidth. 6 The modulation described in this letter uses a tone spacing of 210 MHz for a laser operating at 10 mW output power with a 10 MHz linewidth. Experimental: The FSK modulator uses injection-current fre- quency tuning and, in the present case, is designed to provide equal injection current increments between adjacent frequency tones. The design of the modulator is modular and consists of a 0 to 200 mA variable constant current source to bias the diode and define the frequency of tone 0. A series of high- speed GaAs FETs switch bipolar constant current sources to provide the increments in injection current that define the frequencies of tones 1, 2 and 3. The currents sum in a bias network to form a modulated injection current. The bias network also provides some degree of compensation for the low frequency increase in the frequency modulation response of the laser diode due to thermal tuning mechanisms. 2 The modulator was designed to be driven from a 2 bit binary data source via a simple hard-wired binary to 4-ary data translator. The architecture of this translator governs the relationship between the source data and the transmitted fre- quency tone, and could be used to partially code the transmit- ted data. The modular architecture of the modulator design allows flexibility in system configuration and expansion. For example, a 4-ary modulator could be configured with three switched current sources, each of a different value; with three switched current sources, each of the same value; or with two switched current sources that have binary weighting. Expan- sion to an M-ary modulator requires M — 1 independent current sources, or log 2 M binary weighted current sources. The present modulator uses three different current sources, so only one current source is on at any instant. A schematic diagram of the experimental arrangement is shown in Fig. 1. The laser diode was a 300 /mi Hitachi CSP laser operating in a single longitudinal mode at 830 nm. The temperature of the diode was stabilised to better than 1 mK using thermoelectric elements. The diode was operated at l-8/ tfc with an output power of 10 mW and a linewidth of 10 MHz. A 1-5 GHz free spectral range scanning confocal Fabry-Perot interferometer, with a resolution of 5 MHz, was used to monitor the FSK tone spacing. The 4-ary FSK signal was temporally measured using an optical discriminator in the form of an unbalanced Michelson interferometer. The interfer- ometer had a 270 ps optical path delay and was frequency locked to the mean frequency of the laser by an integrating feedback loop. The operating point of the interferometer was maintained at quadrature. The interferometer output was detected by an APD, amplified and displayed on an oscillo- scope. The measurement system rise time was estimated to be 1-5 ns. Fabry-Perot I interferometer! temperature controller ser diode and " nator F bias j „ j current I • ima I / Michelson interferometer bias network amplifier photodetector FSK I iulator I oscilloscope I spectrum analyser Fig. 1 Schematic diagram of the experiment arrangement Data for the modulator was provided by a data generator in the form of two independent, repetitive, 16 bit sequences at a rate of 100 Mbit/s (50 Msymbols/s). Fig. 2 shows 10 bits of the two 16 bit sequences and the corresponding output of the 4-ary FSK modulator into a 50 Q load. The modulator rise bit 0 bit 1 binary input data - ary modulator output Fig. 2 Binary data inputs (upper two traces) and the modulator output (lower trace) Timescale 20 ns/div 12 ELECTRONICS LETTERS 3rd January 1985 Vol.21 No. 1