Gain-switched Distributed Feedback Laser Diode Research Articles

Evaluation of the phase randomness of a light source in quantum-key-distribution systems with an attenuated laser

The phase randomized light is one of the key assumptions in the security proof of Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol implemented with an attenuated laser. Though the assumption has been believed to be satisfied for conventional systems, it should be reexamined for current high speed QKD systems. The phase correlation may be induced by the overlap of the optical pulses, the interval of which decreases as the clock frequency. The phase randomness was investigated experimentally by measuring the visibility of interference. An asymmetric Mach-Zehnder interferometer was used to observe the interference between adjacent pulses from a gain-switched distributed feedback laser diode driven at 10 GHz. Low visibility was observed when the minimum drive current was set far below the threshold, while the interference emerged when the minimum drive current was close to the threshold. Theoretical evaluation on the impact of the imperfect phase randomization provides target values for the visibility to guarantee the phase randomness. The experimental and theoretical results show that secure implementation of decoy BB84 protocol is achievable even for the 10-GHz clock frequency, by using the laser diode under proper operating conditions.

Gain-switched multicarrier transmitter in a long-reach UDWDM PON with a digital coherent receiver

The authors report on the downlink performance of a 10 Gb/s long-reach and ultra-dense wavelength-division multiplexed passive optical network, based on a multicarrier transmitter realized by using an externally injected gain-switched distributed-feedback laser diode. Each of the comb channels, spaced by 10 GHz, is modulated with a 3 Gbaud dual polarization quadrature phase shift keying signal that included a 20% overhead for forward error correction. Frequency selectivity and enhanced receiver sensitivity is achieved by employing a digital coherent receiver to receive the signal. Experimental results achieved in a back-to-back and 100 km transmission scenarios show an excellent worst case receiver sensitivity of -44 dBm.

Distinguished RZ-OOK Performances Between DFBLD Pulsed Carriers Self-Started by Gain Switching and Nonlinear Absorption Modulation

The distinguished transmission performances of the return-to-zero (RZ) on-off-keying data stream at 10 Gb/s carried on the nonlinearly modulated and gain-switched distributed feedback laser diode (DFBLD) pulse train triggered with a self-feedback optoelectronic oscillator (OEO) are compared. The OEO driven gain-switching DFBLD with insufficient pulse extinction is not particularly suitable for RZ data generation owing to its intense relative intensity noise (RIN) and large timing jitter under threshold operation. In contrast, the nonlinearly driven electroabsorption modulator delivers a self-pulsated RZ carrier with smaller timing jitter of 600 fs, shorter pulsewidth of 21 ps, higher pulse on/off extinction ratio up to 9.8 dB, and a better receiving power sensitivity of -20 dBm for bit error rate <;10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> as compared to the gain-switched DFBLD based RZ carrier. In comparison with the gain-switching technique, the nonlinear absorption modulation greatly reduces the peak-to-peak frequency variation and chirp parameter of the self-pulsated RZ carrier to -12 GHz and -0.32, respectively.

A novel method of chirp elimination using reconstruction equivalent chirp superstructured fiber Bragg grating

A novel chirp elimination and pulse compression method is firstly proposed using reconstruction equivalent chirp superstructured fiber Bragg grating（REC-SSFBG）. Based on the feasibility of REC technology for any physical realizable target response， this novel chirp elimination method is suited for any model of chirped pulse. The simulation results show that the time bandwidth product decrease from 225， 265 and 250 to 0458， 0636 and 073 for linearly chirped， Gauss chirped and Lorentz chirped pulses， respectively. The original pulsewidth is 20 ps and the chirp coefficients is -5 for all the three cases. Further experiment is performed with a fabricated REC-SSFBG to compress a chirped pulse generated from commercial software with model of gain-switched distributed feedback laser diode. The pulsewidth decreases from 25 ps to 5 ps， which agrees well with the simulation.

Generation of ultrawideband pulses using a distributed fiber-link system

This work develops a simple distributed fiber-link system with a newly designed antenna to generate ultrawideband (UWB) pulses for indoor wireless communications. The proposed system comprises a gain-switched distributed feedback laser diode (DFB-LD), electro-absorption modulator (EAM), a photodetector and a pair of coplanar waveguide (CPW) antennas. This optical approach can tolerate pulse dispersion and prevent the generation of intersymbol interference (ISI), since the optical pulse width is easily adjustable, providing better radio communication, as determined by the generation of UWB signal. Furthermore, the simulated and experimental results demonstrate that the proposed system can generate high-quality UWB pulse signals for application to future indoor high-bit-rate wireless communications.

The time-window of pulse injection seeding for effective spectra linewidth narrowing of gain-switched DFB laser diodes

Spectral linewidth narrowing of gain-switched distributed feedback laser diode with pulse injection seeding is related to injection time, as indicated by experiment and theoretical analysis. The experimental and numerical simulation results show that in an approximately 100 ps time-window before the generation of optical pulse, the spectral linewidth can be narrowed dramatically by injecting pulse seeding, and that spectral linewidth decreases with increasing intensity of injection pulse. Near transform-limit low-chirp ultrashort optical pulses can be obtained with optical pulse injection seeding. A spectral linewidth reduction from 0.46 to 0.08 nm was obtained and the time-bandwidth product of 2.46 lowered down to 0.70 correspondingly in the experiment.

Fast spectral improvement in picosecond pulses generated from a DFB laser diode using a loosely coupled external cavity

A simple external cavity is constructed to provide adjustable feedback to a gain-switched distributed feedback laser diode. A 27-dB enhancement in the sidemode suppression ratio (SMSR) and a 0.3-nm reduction in the spectral linewidth have been observed. The improvement shows a saturation behavior when the feedback power reaches about -16 dBm. We report on the evolution and the dynamics of the spectral improvement. The steady state can be achieved as soon as four round-trip feedback cycles are completed. Dependences of the speed of improvement and the SMSR on the feedback pulse arrival time are also investigated. The optimal result can be obtained over a sensitive time window of about 20 ps.

Chirp elimination using a linearly chirped fiber grating

Gain-switched distributed feedback laser diodes (GS-DFB-LD) with high repetition bit rate have large frequency chirp. Conventionally, the F–P spectrum filter and normal dispersion fiber were used to eliminate chirp in GS-DFB-LD pulse sources. In this paper, a new method for eliminating chirp of high repetition pulses generated from DFB laser diodes is proposed. The use of a linearly chirped fiber grating for chirp elimination is discussed theoretically and numerically. A new apodized coupling profile function for the grating, which we call it the inverse Gaussian, is given. Its characteristics are analyzed and compared with other profiles. The numerical results are in agreement with the theoretical analysis.

Method of generating nearly transform-limited pulses from gain-switched distributed-feedback laser diodes and its application to soliton transmission

A train of nearly transform-limited pulses are generated from a 1.55-microm distributed-feedback laser diode by using gain switching in a novel technique, which is a combination of a spectral window and temporal compression. By amplifying these pulses with erbium-doped fiber amplifiers, we demonstrate soliton transmission in a conventional single-mode fiber.

Adiabatic soliton compression of gain-switched DFB-LD pulse by distributed fiber Raman amplification

Using a dispersion-shifted single-mode fiber pumped by laser diodes as a distributed fiber Raman amplifier, the authors adiabatically compressed transform-limited (TL) pulses generated by a gain-switched distributed-feedback laser diode followed by linear compression and a spectral window. The initial TL pulses were successfully compressed asymptotically to almost pure solitons as short as 3.6 ps at the 2.4 dB total gain of a distributed fiber Raman amplifier, even without using solitons as the initial pulses. The experimental results are well supported by numerical calculations made with the beam propagation method. >

Nonlinear interaction between optical pulses from high-repetition gain-switched DFB-LD in optical fiber

Interaction forces between propagating pulse pairs from a gain-switched distributed feedback laser diode (DFB-LD) were investigated. The authors discuss the experimental observation of a nonlinear interaction force between the fiber-compressed, gain-switched optical pulses. Optical pulses were generated by a gain-switched 1.55 mu m DFB-LD at a 10 GHz repetition rate, and compressed by compensating for the red-shifted chirping in the optical pulses using a polarization-holding fiber with a 18.7 ps/km/nm normal dispersion at a 1.55 mu m wavelength. The measured interaction force, which was greater than that theoretically predicted, was caused by the wings and/or pedestal associated with copropagating pulses.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transform-limited pulse generation in the gigahertz region from a gain-switched distributed-feedback laser diode using spectral windowing

For the first time, to our knowledge, transform-limited and high-power picosecond pulses in the gigahertz region are successfully generated from a distributed-feedback laser diode using spectral manipulation with a narrow-band Fabry-Perot resonator and erbium fiber amplifiers. The pulses, which before the optical filtering have a width of 27 psec and a spectral width of 1.24 nm, are transformed to transform-limited pulses with a width of 17 psec and a spectral width of 0.21 nm. The peak power of the pulse at a 6-GHz repetition rate is amplified to greater than 90 mW with an erbium fiber amplifier.

3.2-5 Gb/s, 100 km error-free soliton transmission with erbium amplifiers and repeaters

For the transmissions, the transform-limited soliton pulse source is a gain-switched distributed-feedback laser diode with a narrowband spectral filter and erbium amplifiers. A LiNbO/sub 3/ light intensity modulator is used for pulse switching. The preemphasis technique for sending solitons over a long distance, in which an erbium optical repeater is installed every 25 km as a lumped amplifier, is used. >

Generation of &lt;3 ps optical pulses by fibre compression of gain-switched InGaAsP DFB laser diode pulses

Generation of <3 ps full width at half maximum (FWHM) 1.3 mu m pulses by compressing the chirped output of a gain-switched distributed feedback laser diode (DFB-LD) with a fibre dispersive delay line is reported. Diode laser requirements for minimum pulse width and maximum pulse energy, as well as suppression of 'wings' on the optical pulse, are discussed.

Dynamic optical soliton communication

Digitally coded optical solitons at 5 and 10 Gb/s have been successfully transmitted over 400 and 300 km, respectively, using erbium-doped fiber amplifiers and repeaters. The soliton pulse source is a gain-switched distributed-feedback laser diode with spectral windowing. The repeater spacing for the 10-Gb/s transmission with an input soliton of A=1.4 is 25 km, which is extended to 50 km for 5-Gb/s transmission with an input soliton of N=1.8-2.0. >

Demonstration of chirping manipulation of laser diode chirped optical pulses utilising cross-phase modulation in optical fibres

A new method to manipulate the chirp characteristics of laser-diode pulses is proposed, utilising the cross-phase modulation in optical fibres. Chirping manipulation of a redshift chirped pulse from a 1.3 µm gain-switched distributed feedback laser diode is first demonstrated by compensating for the residual chirp through the blue-shift chirp induced by a 1.3 µm intense YAG pump pulse.

100Gbit/s optical signal generation by time-division multiplication of modulated and compressed pulses from gain-switched distributed feedback (DFB) laser diode

An all-optical spatial time-division multiplexer is demonstrated using fibre-type 2 × 2 couplers and fibre delay lines. 100Gbit/s modulated optical signals are successfully generated by 32-times multiplication of 3.125Gbit/s, 6ps compressed pulses, from a gain-switched DFB laser diode.

Ultrafast all-optical switching utilizing the optical Kerr effect in polarization-maintaining single-mode fibers

An ultrafast all-optical switching scheme utilizing the optical Kerr effect in two birefringent fibers concatenated with each fast axis crossed is proposed. Stable optical Kerr modulation and all-optical demultiplexing of an ultrashort (30 p.s.) optical pulse train at 1.97 GHz from a gain-switched distributed-feedback laser diode (DFB LD) have been sufficiently demonstrated using CW mode-locking Nd:YAG laser pulses as a pump. Switching speed and required pump powers are studied in terms of fiber bandwidth due to fiber birefringence, and combined effects of chromatic and polarization dispersions on Kerr modulation profiles. By utilizing the optical Kerr modulation properties in the presence of dispersions, the nondiagonal yx component of the nonlinear refractive index is also determined to be 0.34 relative to the diagonal component xx. The intrinsic stability and ultimate switching capabilities are discussed. >

High-speed picosecond optical pulse compression from gain-switched 1.3-µm distributed feedback-laser diode (DFB-LD) through highly dispersive single-mode fiber

Picosecond optical pulse compression characteristics of chirped pulses from gain-switched distributed feedback-laser diodes (DFB-LD) transmitting through highly dispersive media are studied theoretically and experimentally. It is clarified theoretically that gain-switched chirped pulses can be compressed to about a 0.7-time bandwidth product by normal dispersion of the dispersive media and that the optimum dispersion value to obtain a minimum compressed pulse is proportional to the square of original pulsewidth. Through a dispersion, shifted single-mode fiber with -48-ps/nm normal dispersion at a 1.3-μm wavelength, gain-switched 30-ps (FWHM) pulses from a directly modulated 1.3-μm DFB-LD at a 4.4-GHz repetition rate have been successfully compressed to 6.4-ps optical pulses with a 0.86-time bandwidth product. Experimental results agree with the theoretical analysis.

Transform-limited 5.6 ps optical pulse generation at 12 GHz repetition rate from gain-switched distributed feedback laser diode by employing pulse compression technique

12 GHz chirped optical pulses from a gain-switched 1.3 μm distributed feedback laser diode are compressed to transform-limited 5.6 ps pulses by travelling through a dispersion shifted single-mode fibre with 46 ps/nm normal dispersion. A time-bandwidth product of 0.69 and a high on/off ratio of 20 dB are obtained for optimally compressed pulses.