Semiconductor laser stabilization by external optical feedback

A report is presented on a general theory describing the effect of external optical feedback on the steady-state noise characteristics of single-mode semiconductor lasers. The theory is valid for arbitrarily strong feedback and arbitrary optical feedback configuration and spectrum. A generalized Langevin rate equation is derived. The equation is, in general, infinite order in d/dt constituting an infinite-order correction to the low-frequency weak-feedback analysis. The general formalism includes relaxation oscillations and permits analysis of the effect of feedback on the laser linewidth, frequency noise, relative intensity noise, and the relaxation oscillation sidebands in the field spectrum. The theory is applied to two important feedback configurations: the laser coupled to a single mirror and the laser coupled to a high-Q cavity     

Ultrafast operation of optically pumped resonant periodic gain GaAssurface emitting lasers

Generation and characterization of picosecond optical pulses from vertical-cavity resonant-periodic-gain GaAs-AlGaAs surface-emitting lasers optically pumped by picosecond dye-laser pulses is reported. The output pulseshape was obtained from the cross correlation of pump and signal pulses. Dependence of signal pulsewidth and pulse delay on pump power were investigated. The results are in good agreement with a single rate equation model of the pulse formation. A cavity lifetime of 8.3 ps, compared with a gain medium transit time of ~0.1 ps, is determined for this very high-Q structure     

Experimental and theoretical investigation of large output powerQ-switched AlGaAs semiconductor lasers

The response of AlGaAs two-section broad-area lasers to picosecond optical excitations was investigated. Q-switched pulses of up to 6 W amplitude with pulse widths less than 30 ps were measured. Asymmetric Q-switched behavior was theoretically predicted and excellent agreement with experimental results achieved on time scales shorter than the cavity round-trip time     

Self Q switching of a CW mode-locked Nd:YLF laser bycavity length detuning

The cavity length of a CW mode-locked Nd:YLF laser is detuned to investigate the effect of detuning on the output of the laser. Negative detunings of 0.4 to 2.4 μm result in periodic modulations of the pulse envelope. Further negative detunings of 2.4 to 3.4 μm produce undamped regular spikings, that is, stable self-Q-switching behavior with mode-locked pulses of 30~47 ps pulsewidths, which are as short as those of plain mode locking without Q switching. Similar self-Q-switching behavior is also produced by positive detunings, but the stability is very poor     

Mode-locked hybrid soliton pulse source with extremely wideoperating frequency range

The authors report a mode-locked pulse source with extremely wide operating frequency range and very stable operation, through the use of a long, linearly chirped Bragg reflector as the output coupler integrated in a fiber external cavity. A 1.55 μm strained MQW laser diode is used, with one facet high reflectivity (HR) coated for improved cavity Q, and the other antireflection (AR) coated to allow coupling to the external cavity and suppress Fabry-Perot modes. Near-transform-limited pulses are obtained over a frequency range of 700 MHz around a system operating frequency of 2.488 GHz, with pulsewidths of 50 ps, as required for a practical soliton transmission system     

Development of EM field in lasers with rotating mirrorQ-switch

A fast-Fourier-transform (FFT) computational scheme based on a Fourier expansion technique is developed to calculate the development of an electromagnetic (EM) field from spontaneous noise inside a loaded rotating mirror Q-switched laser. A rate equations analysis is also carried out, using the FFT-calculated dependence of the diffraction loss on the fixed Q-switch mirror tilt angle. The computational results are compared with the output characteristics of an eye-safe (λ=1.54 μm) erbium glass laser. The experiments are in good qualitative and quantitative agreement with the predictions of the FFT mode. The model reveals short-duration intense spatial regions, as well as two types of temporal modulations of the laser output pulses. These modulations are attributed to the inhomogeneous development of the laser field in the longitudinal and transverse laser resonator direction. These irregularities must be taken into account when calculating eye-safety levels of rotating mirror Q-switched lasers     

Static and dynamical characteristics of narrow-band tunableresonant amplifiers as active filters and receivers

The static and dynamical characteristics of semiconductor narrowband tunable resonant optical amplifiers as active filters and receivers are theoretically studied. Device performance at different cavity length, electrical bias level, incoming signal level, frequency detuning, and signal bandwidth is calculated for both amplitude-shift keyed and frequency-shift keyed input. Nonlinear dynamical effects under high-Q conditions and high input power are also discussed. Through the coupled field and carrier dynamics, the amplifier response can be monitored by the amplifier terminal voltage. The calculations are compared with experimental results where the amplifier is used as tunable receiver     

Picosecond injection laser: a new technique for ultrafastQ-switching

A modification of a method for self-Q-switching in a laser with an isotypic saturable absorber is suggested and implemented in a multisegmented injection laser. The method consists of the transportation of most parts of the excited absorber population to the amplifier. The carrier transport time must be less than the spontaneous recombination time in the absorber. In a three-section AlGaAs/GaAs double-heterostructure laser with modified Q-switching, optical pulses of 5 ps in duration with a repetition rate as high as 18.5 GHz and peak power above 10 W have been obtained. The latter value is the largest ever reported for a picosecond injection laser. Unique temporal and spectral features exhibited by these lasers have been observed, including the stepped variation of pulse repetition frequency, its dependence on the pump current, large emission spectral width (on the order of kT), spectral chaos and bistability     

An automated 60 GHz open resonator system for precision dielectricmeasurements

An automated open resonator system was designed and constructed for precision measurement of the loss tangent and dielectric permittivity of low absorbing materials at 60 GHz. The use of a high-Q hemispherical Fabry-Perot cavity together with highly stabilized synthesized phase-locked Gunn oscillator sources and a superheterodyne receiver made it possible to measure loss tangent values as low as 10 μrad. Both cavity length variation and frequency variation techniques were utilized to provide precise data     

Integrated Q-switched multiple-cavity glass waveguidelaser

A novel Q-switching scheme, using rapid variation of the path difference between the cavities of a multiple-cavity resonator, is described. A thermooptic phase modulator was used to switch the cavity loss of a Y-junction glass waveguide laser between high and low states. Q-switched pulses with durations of 5 μs and peak powers of 70 mW were obtained     

Short pulse injection seeding of Q-switched Nd:glass laseroscillators-theory and experiment

A 0.5-GW-peak-power solid-state laser source that is based on injection seeding a Q-switched Nd:Glass laser is discussed. In the first experimental demonstration, a Q-switched oscillator producing 101 mJ was seeded by a train of 11-ps pulses from a CW (continuous-wave) mode-locked laser to produce injection-mode-locked pulses under a 91-ns envelope. A theoretical analysis of injection seeding of a high-gain Q-switched oscillator by the output of a mode-locked oscillator is presented. The numerical analysis predicts the minimum signal power required for injection mode locking and the temporal shape of the output pulse. The experimental results agree well with the theoretical predictions. The amplification demonstrated by this technique is 104.4 dB, which is much greater than that demonstrated by a multipass or regenerative amplifier. The experimental advantages of injection mode locking include greater than 100 dB of effective amplification and noncritical cavity length adjustment of the seed resonator     

Optical parametric oscillation in MgO:LiNbO3 driven by adiode pumped single frequency Q-switched laser

Optical parametric oscillation tunable over the 940-1220-nm spectral region has been obtained from a monolithic magnesium-oxide-doped lithium niobate ring resonator. The monolithic optical parametric oscillator (OPO) was pumped by the 532-nm second harmonic of a diode-laser-pumped single-frequency Q-switched Nd:YAG laser. The exceptional frequency and amplitude stability of the single-frequency pump source provided stable OPO output pulses     

Modulation instability and soliton-Raman generation inP2O5 doped silica fiber

Generation of high-repetition-rate modulation instability pulse trains and high- energy soliton-Raman pulses of ≃60 fs durations, in a single-mode P₂O₅-doped silica fiber, is reported. The 7 mol.% concentration P₂O₅ fiber was pumped in a single pass arrangement by a Q-switched and mode-locked Nd:YAG laser operated at 1.319 μm. Operating in the region of zero second-order dispersion, modulation instability Stokes sideband seeded resonantly cascade stimulated Raman scattering associated with Si-O-Si and P=0 vibration modes of the fiber. As a result, broad bandwidth and widely tunable 1.36-1.80 μm femtosecond pulses were obtained     

An EMQ-switched CO2 laser as a pump source for afar-infrared laser with a high peak power and a high repetitionrate

A modified current pulsed Q (EMQ)-switched CO₂ laser which is Q-switched by a mechanical beam chopper in combination with a pulsed discharge current is discussed. The laser produces a very stable output with a peak power greater than 1 kW at a repetition rate of 1000 p.p.s. for all transitions in the P and R branches of the CO₂ spectrum. A CH₃F laser pumped by the EMQ-switched laser produces 496 μm radiation in a 6.5 W peak, 100 ns pulses at 500 p.p.s. in the lowest loss EH₁₁ mode     

Acoustooptic Q-switching of erbium lasers

Experiments were performed on Q switching of erbium lasers at 3 μm with an acoustooptical modulator. Different laser crystals of YAG:Er and YSGG:Cr:Er have been investigated. The highest fraction of single pulse Q-switch energy to free-running mode energy was 1%. Pulse durations of 50 ns were obtained. Multiple switching with up to 14 Q-switch peaks per flashlamp pulse could be generated, containing up to 19% of the energy of the free-running mode     

Electrooptically Q-switched 2.79 μm YSGG:Cr:Er laserwith an intracavity polarizer

Single Q-switched pulses have been generated in YSGG:Cr:Er with a 360-ns-risetime LiNbO₃ electrooptical modulator. It is shown that birefringence losses can be avoided and spiking emission eliminated by placing an additional polarizing LiNbO ₃ prism inside the laser resonator. Reproducible single Q -switched pulses of 200-ns duration have been generated     

Compact system of wavelength-tunable femtosecond soliton pulsegeneration using optical fibers

Using passively mode-locked femtosecond (fs) fiber laser and polarization maintaining fibers, the compact system of wavelength-tunable femtosecond (fs) fundamental soliton pulse generation is realized. The monocolored soliton pulse, not multicolored ones, with the ideal sech² shape is generated, and its wavelength can be linearly shifted by varying merely the fiber-input power in the wide wavelength region of 1.56-1.78 μm for a 75-m fiber. The soliton pulses of less than 200 fs are generated with the high conversion efficiency of 75%-85%. This system can be widely used as a portable and practical wavelength-tunable fs optical pulse sources     

Picosecond pulse generation in a GaAs/GaAlAs single-quantum-welllaser at the first and second subband transition

Picosecond optical pulses are generated in a single-quantum-well laser at the n=2 or n=1 quantized transition by tuning the optical gain spectra via the intracavity losses. The results for the generated pulses are discussed with respect to the influence of differential gain (dg/dN) and nonlinear gain saturation (∈) effects     

Experimental study of additive-pulse mode locking in an Nd:glasslaser

Self-starting additive-pulse mode locking (APM) has been investigated experimentally in a continuous-wave Nd:glass laser. Stress has been put on the study of the self-starting process and relaxation oscillation instabilities. An intensity threshold for self-starting APM is observed and related to the linewidth of the first beat note of the power spectrum of the free-running laser output. Under steady-state conditions, two distinct operating regimes are obtained: mode locking with repetitive self-Q switching and pure mode locking. Increasing the intracavity power gives rise to a strong chirp developing on the mode-locked pulses and to a simultaneous disappearance of self- Q switching     

Self-starting, passively mode-locked Fabry-Perot fiber solitonlaser using nonlinear polarization evolution

The authors demonstrate pure self-starting mode-locked operation in a fiber Fabry-Perot cavity. The laser produces 1.6 ps transform-limited soliton pulses by means of nonlinear polarization evolution in conjunction with an intracavity fiber polarizer. This laser system can be used for the generation and study of soliton pulses in the picosecond regime. With a careful choice of the fiber dispersion and fiber length, the system should also be able to operate in the femtosecond regime. The advantage of the system over existing self-starting fiber soliton lasers is that an optical isolator (an expensive component) is not required. The replacement of the two mirrors with fiber reflection gratings would make this configuration a truly all-fiber device