Data encoded optical pulse generator

Abstract: An apparatus is disclosed for producing encoded optical pulses for transmission in an optical communication system. The apparatus comprises a semiconductor laser (10) and a modulator (30), the laser for providing an optical source, the modulator for modulating the optical source. The laser is biased to produce a CW optical output and RZ electrical data pulses (570) are applied to the modulator to modulate the CW optical output to produce RZ optical data pulses corresponding to the RZ electrical data pulse. The RZ electrical data pulses are formed by performing a gating operation, using a dual gate FET (500), of an NRZ data input (545) with a periodic analog signal, generated by superimposing harmonic related sinusoidal signals to provide electrical RZ data pulses.

Description: BACKGROUND OF THE INVENTION Technical Field This invention relates generally to an optical pulse generator and more particularly to a data encoded optical pulse generator for generating soliton pulses. Discussion of Related Art A useful measure of the performance characteristics of a digital optical data communication system is the well-known "rate-length product", i.e., the product of the system data rate and the length of transmission. It is often a design goal to achieve the highest data rate through the longest repeaterless length and consequently the highest rate-length product. Components used in long distance optical data communication systems for producing a high rate-length product typically include: a light source such as a laser diode; a high speed modulator which modulates the light source at the system bit rate, usually by amplitude modulating the light produced by the laser; a low loss, low dispersion fiber medium; a photodetector such as a p-i-n photodiode or an avalanche photodiode having a high speed response for detecting the transmitted optical signals; and a receiver coupled to the photodetector for amplifying and decoding the received optical signals. Components such as optical amplifiers and repeaters can further extend the transmission length and increase the rate-length product. The system rate-length product is a function of the transmission format as well as of the hardware components used. In current optical communication systems, data is transmitted in non-return-to-zero (NRZ) format, with ones and zeros represented by the presence or absence of light in a given time slot. This format is typically implemented by using a laser to generate a CW light beam, then modulating the light beam with an electro-optic modulator. The modulator may be a separate semiconductor device. Recently, the laser and modulator have been fabricated on a single chip, resulting in an integrated transmitter for NRZ communications systems. The rate-length product attainable with the NRZ transmission format is ultimately limited by dispersion in the optical fiber. To reach very high data rates, communications links employing soliton pulses have been proposed. Solitons are optical pulses which take advantage of the nonlinearity of the fiber to maintain pulse shape during transmission. Soliton pulses can be transmitted over long lengths of fiber at rates of 10 Gb/s and higher. The soliton pulse width is less than the width of the time slot and is thus transmitted in return to zero (RZ) format; i.e., the amplitude of the light returns to zero during each time slot. An RZ format is also desirable for systems employing optical time-division-multiplexing and demultiplexing. Despite recent advances in the development of optical transmitters, a need exists for a transmitter which produces soliton pulses and is wavelength tunable, compact, manufacturable and relatively inexpensive to operate for producing RZ encoded data. One approach is to use a gain-switched laser to generate pulses, then encode data onto the pulses with a modulator. However, gainswitching produces chirp, which causes significant penalties in transmission due to fiber dispersion, and may also degrade the performance of a soliton system. Another approach is to use a mode-locked laser to generate pulses and again encode data with a modulator. However, monolithic mode-locked lasers operate only at fixed frequencies which are determined by their cavity length. The fabrication of devices with a desired operating frequency may be difficult, especially if the laser is to be integrated with the modulator for data encoding. External-cavity mode-locked lasers offer more flexibility in operation frequency, but are bulky and sensitive to the environment, making them unsuitable for practical applications. Integrated laser-modulators previously suitable for NRZ data transmitters can be operated as transmitters for producing soliton pulses. In such case the laser is operated CW and the modulator is driven with an RF sinusoid, resulting in a time-varying transmission which converts the CW laser light to pulses. This type of pulse source is simple, compact, frequency and wavelength tunable, and possesses spectral characteristics suitable for long distance transmissions. A soliton transmitter based on this pulse source, consisting of a laser integrated with two modulators, has been demonstrated. In this approach, the first modulator is used to convert CW laser light to pulses and the second modulator is used to encode data. While this technique is attractive, the integration of a laser and two modulators requiring two high-speed contacts is difficult. A simpler device would be preferable. EP-A-0 566 464 discloses an apparatus and method for producing encoded optical pulses (solitons) for transmission in an optical communication system, comprising a laser and a modulator, means for biasing the laser to produce CW optical output, and means for applying to the modulator an electrical data signal for generating optical data pulses corresponding to the electrical data signal. The electrical data signal is produced for example by using a binary data signal in combination with a sinusoidal signal from an oscillator. FROBERG N M ET AL: 'PULSE GENERATION BY HARMONIC MODULATION OF AN INTEGRATED DBR LASER-MODULATOR', ELECTRONICS LETTERS, vol. 30, no. 8, 14 April 1994, page 650/651, discloses an apparatus and method for producing optical pulses (solitons) for transmission in an optical communication system, comprising a semiconductor laser (DBR laser) and a modulator (InGaAsP quantum well electroabsorption modulator) integrated with the laser, means for biasing the laser to produce CW optical output, and means for applying to the modulator a periodic analog signal which is generated by superimposing harmonic related sinusoidal signals (the modulator was biased at -5V DC and driven with both the 5 and the 10 GHz sinusoids in order to further reduce the optical pulse width generated by the modulator). SUMMARY OF THE INVENTION The invention is as set out in the independent claims. Briefly, according to the present invention, an optical pulse generator, preferably an integrated semiconductor laser-modulator is used to simultaneously generate pulses and encode data in RZ format, eliminating the need for an external modulator and reducing cost, bulk and complexity. The laser is preferably of the Distributed Bragg Reflector (DBR) type and the laser-modulator is variable in pulse width, repetition rate and wavelength. The apparatus comprises: a semiconductor laser and a modulator, the laser for providing an optical source, the modulator for modulating the optical source; means for biasing the laser sufficient to produce a CW optical output; means for applying electrical data pulses to the modulator to modulate the CW optical output to produce optical data pulses corresponding to the electrical data pulses. The electrical data pulses applied to the modulator are in return-to-zero format and the optical data pulses output from the laser-modulator are also in return-to-zero format. The means for applying electrical data pulses to the modulator section includes a device for performing a gating operation of an NRZ data input with a periodic analog signal input to provide electrical RZ data pulses. In a method of the invention for generating soliton pulses from a semiconductor laser and a modulator, the laser is biased for producing a CW laser output, and an electrical RZ data signal is applied to the modulator for transmitting said laser output when an electrical one is present for generating RZ optical data pulses corresponding to said RZ data signal. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the invention may be better understood from the following detailed description when read in conjunction with the following drawings: FIG. 1 is a schematic of apparatus similar to that of the present invention showing details of the integrated laser and modulator; FIG. 2 is a schematic of apparatus useful for understanding the present invention; FIG. 3 shows an eye pattern of the optical pulse output of the semiconductor device according to FIG. 2; FIG. 4A shows a representative pulse of the optical output and FIG. 4B shows the corresponding spectrum; and FIG. 5 is a schematic of the apparatus in accordance with the present invention according to an alternative biasing arrangement to the embodiment as shown in FIG. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a view of a data encoded optical pulse generator similar to that of the present invention. A semiconductor device 100 preferably includes a bulk electroabsorption modulator and a multiple quantum well Distributed Bragg Reflector (DBR) laser. The device 100 may be fabricated conventionally, and it is apparent to one ordinarily skilled in the art that the techniques of the present invention for generating data encoded optical pulses are also applicable to pulse generators having a semiconductor laser and an external modulator. The DBR laser 10 is preferably operated to output pulses at around 1557 µm wavelength but is discretely tunable over discrete wavelengths from around 1557 um to 1563 µm. Laser wavelength tuning can be by carrier injection or by surface layer resistive heating. Both techniques employ the known principle that the wavelength of a semiconductor laser will change with temperature since its refractive index varies with temperature. The tuning capability of the DBR laser according to the present invention is preferably by surface layer resistive heating of a 220 ohm resistor 20 made on the top metalization layer over the grating region 25. Current Ir is selectively applied to resistor 20 for selectively varying the temperature of the device and tune the optical output wavelength. The laser/modulator 100 is biased to output encoded optical data pulses in RZ format 130. As shown in FIG. 1, data pulses are generated by biasing the laser section 10 at near lasing threshold with a constant current Idc and applying digital data from data source 110. The digital data is preferably in NRZ format. The output of the laser 10 is an optical signal corresponding to the electrical signal 140. The modulator 30 is then biased with a constant voltage source Vdc and an analog signal 120 is also applied. The modulator 30 further modulates the laser output. The resultant pulses 130 output from the laser/modulator 100 are in RZ format as shown. The analog signal 120 and the clock for the data source 110 originate from synthesizer 105, which is a high speed oscillator. For illustration, the synthesizer 105 operates at around 2.5 Ghz for the present preferred embodiment. The output of the synthesizer 105 is split by splitter 115 for providing the clock input to the data source 110 and to a phase control 125 for varying the phase of the analog signal 120. According to the invention, the laser-modulator device 100 produces optical RZ data with another biasing technique. Referring to FIG. 2, the laser 10 is biased at 60 mA to output a CW laser beam, which is then modulated by the modulator 30, which is in turn driven by a digital data signal 570 in RZ format, producing an optical RZ data 580 at the output of the laser-modulator device 100. The components used for producing the digital data signal 570 includes a dual-gate FET 500 for gating an NRZ data stream with an analog signal to produce an RZ format signal corresponding to the NRZ data. The gating process is similar to a logical "AND" operation. The dual gate FET may be any conventional dual gate field effect transistor capable of operating at high frequencies such as up to around 15 Ghz. It is apparent to one skilled in the art that any other transistor(s) configured as shown in FIG. 2 can accomplish the same function. The NRZ datastream input to the FET 500 is produced from pattern generator 540, a high speed data source which produces a preset data pattern in NRZ format at a bit rate corresponding to the rate of the clock input. The sinusoidal signal 535, in this case a 2.5 Ghz sinusoidal signal, is output from phase control 530. The 2.5 Ghz sinusoid and the clock for the pattern generator 540 originate from synthesizer 510, which is a 2.5 Ghz oscillator, the output of which is split by splitter 520. One of the outputs of the splitter is used as the clock input to the pattern generator 540 and the other splitter output is input to the phase control 530, which may be a variable delay line, or any equivalent, for adjusting the phase of the 2.5 Ghz sinusoid signal to coincide with the corresponding bit pattern period of the signal output from pattern generator 540. The dual gate FET 500 performs an AND operation of the NRZ data and the 2.5 Ghz sinusoidal inputs to produce an electrical data stream which is the same data input from the data generator but converted to an RZ format. The output of the FET is further amplified by amplifier 550 before it is used to drive the modulator 30 of the laser-modulator device 100. As configured, the CW laser output of laser 10 is modulated by modulator 30 so that optical pulses are output when an electrical "one" is present to produce RZ optical pulses 580. FIG. 3 shows an eye pattern of the optical pulses output from the apparatus according to the described embodiment of the present invention. FIG. 4A and FIG. 4B show a representative output waveform and the corresponding spectrum, respectively. Due to the non linear switching characteristics of the modulator, the optical output pulses are narrower than the input electrical pulses. The pulse width is 85 ps and can be further reduced by adding higher order harmonics to the electrical pulse shaping input. There is little or no chirp added by data encoding with the modulator. FIG. 5 shows a technique for adding higher order harmonics to the electrical pulse shaping circuit in accordance with the present invention. This technique produces an analog signal by superimposing sinusoidal signals which are harmonics of the oscillation signal originating from the synthesizer 510. The splitter 520 is a three way splitter (which may comprise two two-way splitters), with one output being the clock signal for the pattern generator 540, the second output being a first sinusoid signal and the third output being input to a frequency doubler 555 for creating the second harmonic of the first sinusoid. The output from the frequency doubler 555 may be phase adjusted by another phase controller 565 before being combined with the first sinusoid signal by combiner 585. The output of the combiner 585 is a superimposed, periodic analog signal 535 for inputting to the FET 500. It is readily apparent to one ordinary skilled in the art that a comb generator may be used in place of the frequency doubler arrangement to generate a short electrical pulse for shaping the electrical data. The above description is illustrative of the application of the principles of the present invention. Modifications and other arrangements of the invention can be made by those skilled in the art without departing from the scope of the invention as defined by the claims.

Claims: An apparatus for producing encoded optical pulses for transmission in an optical communication system, comprising: a semiconductor laser (10) and a modulator (30), the laser for providing an optical source, the modulator for modulating the optical source; means for biasing said laser to produce a CW optical output; means for applying RZ electrical data pulses (570) to said modulator to modulate said CW optical output to produce RZ optical data pulses (580) corresponding to said RZ electrical data pulses, wherein said means for applying RZ electrical data pulses to said modulator includes a device (500) for performing a gating operation of an NRZ data input (545) with a periodic analog signal (535) to provide said electrical RZ data pulses, said periodic analog signal being generated by superimposing harmonic related sinusoidal signals. The apparatus according to claim 1 wherein said device (500) for performing a gating operation includes a dual gate FET. The apparatus according to claim 1 wherein said periodic analog signal (535) is selectively variable in repetition rate. The apparatus according to claim 1 wherein said apparatus outputs said encoded optical pulses at a selected wavelength and said semiconductor laser further includes means (20) for varying said wavelength. A method for generating soliton pulses from a semiconductor laser (10) and a modulator (30), said modulator being integrated with said laser onto a single semiconductor device, said method comprising: biasing said laser for producing a CW laser output; applying to said modulator an electrical RZ data signal (570) for generating RZ optical data pulses (580) corresponding to said RZ data signal; and combining harmonically related sinusoidal signals for producing an analog periodic signal (535) and gating said analog periodic signal with an NRZ data signal (545) to produce said electrical RZ data signal. A method according to claim 5, wherein said RZ data signal is generated by performing a gating operation of the NRZ data signal (545) at a selected bit rate with the analog periodic signal (535) having a period substantially coinciding with said selected bit rate. A method according to claim 6, further including the step of phase adjusting said analog periodic signal (535) for substantially time coinciding with said NRZ data signal (545) for producing said electrical RZ data signal (570). A method according to claim 5, wherein said electrical RZ data signal (570) is produced by a dual gate FET (500).

Inventors: FROBERG NAM M, GNAUCK ALAN, HANSEN PER BANG, HARVEY GEORGE T, RAYBON GREGORY, FROBERG, NAM M., GNAUCK, ALAN, HANSEN, PER BANG, HARVEY, GEORGE T., RAYBON, GREGORY
Applicants: AT & T CORP, AT&T CORP.
Application year: 1995
Main classification: H04B10
IPC codes: H01S5, H04B10, G02F1