Relative Intensity Noise Measurements Research Articles

20 dBm linearly polarised tunable single-frequencydiode-pumped Er:Yb laser at 1535 nm using anisotropic absorptive intracavity etalon

A linearly polarised single-frequency diode-pumped Er:Yb bulk laser with output power in excess of 100 mW has been demonstrated. The use of an anisotropic absorptive intracavity etalon to achieve both single-frequency operation and linearly polarised output is investigated. Output power and tuning characteristics as well as relative-intensity noise measurements are reported. A new active medium, based on a new glass host with reduced pump-induced birefringence, allowed for negligible performance degradation when using the anisotropic etalon instead of a standard BK7 etalon. A broad tuning range and a relative-intensity-noise peak of about –90 dB/Hz has been achieved.

Measurement and characterization of laser chirp of multiquantum-well distributed-feedback lasers

Measurements of relative intensity noise and modulation response, before and after propagation in optical fiber, of the output field of multiquantum-well distributed-feedback (MQW-DFB) lasers are used to determine the influence of the intraband damping mechanisms, the DFB structure and the carrier transport and carrier capture into the QWs on the laser chirp. The power dependence of the linewidth enhancement factor is shown to explain the saturation of the laser linewidth at high optical powers.

1.3-μm InAsP n-type modulation-doped MQW lasers grown by gas-source molecular beam epitaxy

The effect of n-type modulation doping as well as growth temperature on the threshold current density of 1.3-/spl mu/m InAsP strained multiple-quantum-well (MQW) lasers grown by gas-source molecular beam epitaxy (GSMBE) was investigated for the first time. We have obtained threshold current density as low as 250 A/cm/sup 2/ for 1200-/spl mu/m long devices. The threshold current density per well for infinite cavity length J/sub th//N/sub w/spl infin// of 57 A/cm/sup 2/ was obtained for the optimum n-doping density (N/sub D/=1/spl times/10/sup 18/ cm/sup -3/) and the optimum growth temperature (515/spl deg/C for InP and 455/spl deg/C for the SCH-MQW region), which is about 30% reduction as compared with that of undoped MQW lasers. A very low continuous-wave threshold current of 0.9 mA have been obtained at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest results grown by all kinds of MBE in the long-wavelength region. The differential gain was estimated by the measurement of relative intensity noise. No significant reduction of differential gain was observed for n-type MD-MQW lasers as compared with undoped MQW lasers. The carrier lifetime was also reduced by about 33% by using n-type MD-MQW lasers. Both reduction of the threshold current and the carrier lifetime lead to the reduction of the turn-on delay time by about 30%. The 1.3-/spl mu/m InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay is very attractive for laser array application in high-density parallel optical interconnection systems.

Influence of fiber nonlinearity on the fiber transfer function: theoretical and experimental analysis

The influence of fiber nonlinearity on the fiber transfer function is investigated theoretically and experimentally. A rigorous expression for the fiber transfer function using a directly modulated semiconductor laser as an optical transmitter is presented. Very good agreement of fiber transfer function between experimental data and theoretical predictions is achieved. Furthermore, the fiber parameter values extracted from fitting are the same as those obtained from relative intensity noise measurements, and the fiber nonlinearity coefficient value is compared with other published results. A thorough physical explanation for the effect of fiber nonlinearity on the fiber transfer function at low-medium frequencies is provided. Results reveal that in the anomalous dispersion regime the fiber nonlinearity enhances the high-pass behavior typical of the fiber transfer function using a directly modulated laser operating at high bias current, shifts the transfer function dips toward higher frequency, and consequently leads to a larger bandwidth. Furthermore, it is shown that the fiber nonlinearity can provide partial or total compensation of the nonuniform frequency response at low-medium frequencies resulting from a directly modulated laser operated at low bias current.

Relative intensity noise and linewidth measurements of a widely tunable GCSR laser

The intensity and frequency noise of a grating assisted codirectional coupler with rear sampled grating reflector laser with 40-nm tuning range and 35-dB sidemode suppression ratio (SMSR) have been measured. The linewidth varies from 430 kHz to 4 MHz as a function of tuning. The resonance frequency, intrinsic bandwidth and damping factor for this device were extracted from the relative intensity noise (RIN) measurements. At high bias, the RIN is -150 dB/Hz and the maximum resonance frequency is 4.5 GHz.

Tensile-strained GaInAsP-InP quantum-well lasers emitting at 1.3 μm

Tensile-strained GaInAsP-InP quantum-well (QW) lasers emitting at 1.3 /spl mu/m are investigated. Low-pressure metalorganic chemical vapor deposition (LP-MOCVD) is used for crystal growth. High-resolution X-ray diffraction shows good agreement with theoretical simulation, photoluminescence spectra have good energy separation between light-hole and heavy-hole bands due to biaxial tension. The lowest threshold current density for infinite cavity length J/sub th//N/sub w//sup /spl infin// of 100 A/cm/sup 2/ is obtained for the device with -1.15% strain and N/sub w/=3. The amount of strain which gives the lowest J/sub th//N/sub w//sup /spl infin// experimentally clarified is around -1.2%. Threshold current of a buried-heterostructure (BH) laser is reduced to be as low as 1.0 mA. Enhanced differential gain of 7.1/spl times/10/sup -16/ cm/sup 2/ is also confirmed by measurements of relative intensity noise. Much improved threshold characteristic with the feasibility of submilliamp threshold current can be achievable by optimizing the BH structure. The tensile-strained QW laser emitting at 1.3 /spl mu/m with very low power consumption is attractive for the light source of fiber in the loop system and optical interconnection applications.

Operation of diode laser pumped Tm/sup 3+/ ZBLAN upconversion fiber laser at 482 nm

Lasing at 482 nm is observed in Tm/sup 3+/-doped ZBLAN glass fiber pumped with single-mode InP semiconductor diode lasers. Up to 5 mW of 482 nm light is obtained with <40 mW of absorbed pump power from a single 1135 nm pump diode laser. The optimum pump wavelength is measured to be 1135-1340 nm. More efficient laser operation is observed in fiber with 2500 ppm Tm/sup 3+/ compared to 1000 ppm Tm/sup 3+/ because of the reduced length of the fiber laser cavity possible with increased doping. Improved slope efficiencies are also demonstrated when the fiber laser is co-pumped with up to 5 mW from a 1220 nm diode laser. The relative intensity noise (RIN) of the fiber laser displays a maximum of -90 dB/Hz at relaxation oscillation frequencies of a few tens of kHz. The measurement of RIN is limited by shot-noise of -152 dB/Hz above 2 MHz. At higher frequencies, self mode-locking was observed in the fiber laser, which may indicate the existence of saturable absorbers in the fiber core. The presence of such bleachable absorbers is indicated by the observed increase in threshold after upconversion lasing at 482 mm.

Injection locking semiconductor lasers with phase conjugate feedback

Phase conjugate injection locking is shown to couple coherently two semiconductor lasers through a double phase conjugate mirror geometry. The mutual coherence and stability of the lock band are characterized from the visibility and relative intensity noise measurements for various levels of phase conjugate coupling and frequency detunings. Side mode phase conjugate injection locking is demonstrated by high mutual coherence with visibility ∼0.93 and detunings up to nine longitudinal mode spacings at injection levels of −22 dB. Central mode phase conjugate injection locking with high mutual coherence is also possible for feedback levels of −36 dB and exhibiting an asymmetric stable locking region of 350 MHz bandwidth within the locking band. The phase locking is unstable though, the reasons for which are discussed. Increased phase conjugate feedback extends the lock band to greater detunings and asymmetry as in direct injection locking with only a small region showing high mutual coherence. Intensity noise measurements are also discussed.

Experimental study of Auger recombination, gain, and temperature sensitivity of 1.5 mu m compressively strained semiconductor lasers

The effect of strain on Auger recombination has been studied using the differential carrier lifetime technique in both lattice matched InGaAs-InP and compressively strained quaternary quantum wells. It is found that Auger recombination is reduced in strained devices. The transparency carrier density and differential gain of both lattice matched and strained devices have been obtained by gain and relative intensity noise measurement. A reduction of the transparency carrier density is observed in the strained device. However, no differential gain increase is seen. The temperature sensitivity of the threshold current density of both lattice matched and strained devices has been fully studied. Physical parameters contributing to the temperature sensitivity of the threshold current density have been separately measured, and it is shown that the change in differential gain with temperature is a dominant factor in determining the temperature sensitivity of both lattice matched and strained devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Control of differential gain, nonlinear gain and damping factor for high-speed application of GaAs-based MQW lasers

Utilizing small-signal direct modulation and relative intensity noise measurements, the authors investigate changes in the modulation response, the differential gain delta g/ delta n, the nonlinear gain coefficient in , and the damping factor K, which result from three structural modifications to GaAs-based multiple quantum well lasers: the addition of strain in the quantum wells; and increase in the number of quantum wells; and the addition of p-doping in the quantum wells. These modifications are assessed in terms of their potential for reducing the drive current required to achieve a given modulation bandwidth, for increasing the maximum intrinsic modulation bandwidth of the laser, and for improving the prospects for monolithic layer/transistor integration. It has been possible to simultaneously increase delta g/ delta n and decrease K, yielding very efficient high-speed modulation (20 GHz at a DC bias current of 50 mA) and the first semiconductor lasers to achieve a direct modulation bandwidth of 30 GHz under DC bias.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Large and small signal dynamics of vertical cavity surface emitting lasers

Continuous wave relative intensity noise (RIN) measurements and pulsed electrical gain switching measurements were performed on vertical cavity surface emitting lasers in order to investigate the high speed potential of these devices. A high resonance frequency–square root of power coefficient of 7.6 GHz/√mW was obtained from the RIN measurements. The gain switched pulses exhibited very fast relaxation oscillations with a maximum observed relaxation oscillation frequency of 71 GHz.

Effect of Zn doping on differential gain and damping of 1.55 μm InGaAs/InGaAsP MQW lasers

The differential gain and nonlinear damping in 16 well MQW lasers has been studied using relative intensity noise (RIN) measurements. Zinc doping the active region increases the differential gain by 50% to 10−19 m2 and reduces the nonlinear damping factor by 20% to 0.20 nS. This represents a maximum intrinsic 3 dB bandwidth of 45 GHz.

Measurements of relative intensity noise (RIN) in semiconductor lasers

The intensity fluctuations of the laser diodes light are characterized by the so-called relative intensity noise (RIN). In this paper a very accurate measurement technique is presented and results are given on some components. RIN measurements on isolated longitudinal modes can explain how the energy is shared between the modes.

Resonance frequency, damping, and differential gain in 1.5 mu m multiple quantum-well lasers

A systematic investigation is presented into the intrinsic frequency response of quantum-well lasers, using parasitic-free relative intensity noise (RIN) measurements. There is shown to be a strong dependence of the resonance frequency on the number of quantum wells in the active region, originating from variations both in internal losses and in differential gain. The differential gain is found to have values higher than in corresponding bulk lasers, but only in devices with a large number of wells. The damping is also found to vary in a manner consonant with the changes in differential gain; however, comparison with bulk lasers indicates substantially stronger gain suppression in the quantum-well lasers studied. >

Measurements of low frequency and high frequency relative intensity noise in DBR lasers

The relative intensity noise in DBR lasers has been measured as a function of the grating current in both the low frequency and high frequency regimes. With increasing grating current, large increases were only observed in the low frequency intensity noise (&lt;700 MHz). The high frequency intensity noise (&gt;2GHz) was observed to be independent of the grating current.

Determination of the instantaneous linewidth of a monolithic Nd:YAG laser from RIN measurements

The author demonstrates the evaluation of the subhertz instantaneous linewidth of a nonplanar monolithic Nd:YAG ring laser emitting at 1064 nm with 3.9 mW output power from a precise heterodyne relative intensity noise (RIN) measurement. The relaxation resonant frequency, the maximum RIN density at the relaxation resonance, and the relaxation peak width are: 220 kHz, -83 dB/Hz, and 4.5 kHz, respectively, which yield an instantaneous linewidth of 80 mHz. The result is in good agreement with those obtained by other methods.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measured feedback-induced intensity noise for 1.3 µm DFB laser diodes

An experimental set-up has been described, allowing for reliable relative intensity noise (RIN) measurements for a wide feedback range. It could be verified experimentally for DFB lasers that the transition to the coherence collapic regime occurs at significantly larger feedback levels, if the laser is operated at larger optical powers. Furthermore, it has been noted that a large coupling strength κL in DFB lasers is beneficial for improved stability against optical feedback.