Laser Amplitude Noise Research Articles

Noise suppression of a differential detector under high levels of illumination, relevant to terahertz electro-optic sampling

In electro-optic detection of terahertz transients, the photodiodes of a differential detector are often illuminated with optical powers on the order of several milliwatts. We present measurements that show that the response of the photodiodes at these power levels is distorted, giving rise to frequency-dependent amplitude and phase changes in the electrical signal from these diodes. We find that these distortions significantly reduce the ability of the detector to suppress laser amplitude noise. Surprisingly, this occurs at power levels at which the dc output of each photodiode shows no sign of saturation. Based on our measurements, we provide experimenters with recommendations to improve the amplitude noise suppression of a differential detector under high levels of illumination.

LIGO laser intensity noise suppression

In order to meet the strain sensitivity requirements of the Laser Interferometer Gravitational Wave Observatory (LIGO), the laser amplitude noise must be reduced to a level of 1 × 10−8 δP/P, in the bandwidth between 40 and 100 Hz, before injection into the interferometer. This paper describes the control system implemented to meet this goal.

Pump noise as the source of self-modulation and self-pulsing in Erbium fiber laser

We report on the first experimental observation of an influence of noise in a pumping diode laser on the appearance of self-induced modulation and self-pulsing in an all-fiber Erbium laser. The diode laser amplitude noise, being a superposition of spectral components, with some of them to resonantly match the frequency of relaxation oscillations in the Erbium fiber laser at the pump rate fixed, acts as a disturbing force, being a source of the modulation effects. This disturbance enforces the relaxation oscillations of the Erbium fiber laser to become non-dumped, which appears as the self-modulation and even self-pulsing of the laser output power.

Adaptive photodetectors for vibration monitoring

We present characteristics of laser vibrometer using semiconductor GaAs and molecular SnS2 adaptive photodetectors (AP) based on the effect of the non-steady-state photoelectromotive force. AP enable efficient direct conversion of high-frequency phase modulation of speckle-like optical wave reflected from the vibrating object into an output electrical signal with concomitant setting of optimal operation point of the interferometer and suppression of amplitude laser noise. The sensitivity of the setup is analyzed and further improvements in operation of AP are discussed.

Submillimeter wave generation through optical four-wave mixing using injection-locked semiconductor lasers

This paper presents a comprehensive analysis of submillimeter wave generation through optical four-wave mixing (FWM) in an injection-locked semiconductor laser. The probe wave introduces pump-probe difference frequency amplitude modulation (AM) and phase modulation (PM) of the slave laser injection locked to the pump wave. The AM and PM indexes of the locked laser output lightwave modulated at the submillimeter wave frequency have been calculated. The submillimeter wave power generated through this technique has also been calculated theoretically. The analysis predicts a submillimeter wave power of -3.7 dBm at 300-GHz frequency for a free-running slave laser output power of 30 mW. In the presence of amplitude modulation of the probe wave, the same modulation is transferred to the submillimeter wave. The suppression of output lightwave amplitude noise relative to the pump, probe, or free-running slave laser amplitude noise has also been estimated in this analysis. At a submillimeter wave frequency of 300 GHz and a probe power equal to the pump power, typical amplitude noise reduction occurs by 7 dB.

High-dynamic-range laser amplitude and phase noise measurement techniques

We describe techniques for making sensitive and high-dynamic-range measurements of laser amplitude and envelope phase noise (timing jitter) in the frequency domain at the shot-noise limit. Examples of amplitude noise measurements on continuous-wave argon-ion and diode-pumped solid-state lasers used for pumping a femtosecond Ti:sapphire laser are presented. Amplitude and phase noise measurements for the Ti:sapphire laser are also presented, showing correlation between pump laser amplitude modulation (AM) spectra and the resulting AM and phase noise. Characteristics of the measurement system components are discussed, along with examples of the impact these have on achieving reliable high-dynamic-range measurement capability.

Optically sampled analog-to-digital converters

Optically sampled analog-to-digital converters (ADCs) combine optical sampling with electronic quantization to enhance the performance of electronic ADCs. In this paper, we review the prior and current work in this field, and then describe our efforts to develop and extend the bandwidth of a linearized sampling technique referred to as phase-encoded optical sampling. The technique uses a dual-output electrooptic sampling transducer to achieve both high linearity and 60-dB suppression of laser amplitude noise. The bandwidth of the technique is extended by optically distributing the post-sampling pulses to an array of time-interleaved electronic quantizers. We report on the performance of a 505-MS/s (megasample per second) optically sampled ADC that includes high-extinction LiNbO/sub 3/ 1-to-8 optical time-division demultiplexers. Initial characterization of the 505-MS/s system reveals a maximum signal-to-noise ratio of 51 dB (8.2 bits) and a spur-free dynamic range of 61 dB. The performance of the present system is limited by electronic quantizer noise, photodiode saturation, and preliminary calibration procedures. None of these fundamentally limit this sampling approach, which should enable multigigahertz converters with 12-b resolution. A signal-to-noise analysis of the phase-encoded sampling technique shows good agreement with measured data from the 505-MS/s system.

Analysis of light noise sources in a recycled Michelson interferometer with Fabry–Perot arms

We present a method by which the effect of laser field variations on the signal output of an interferometric gravitational wave detector is rigorously determined. Using the Laser Interferometer Gravitational Wave Observatory (LIGO) optical configuration of a power recycled Michelson interferometer with Fabry-Perot arm cavities as an example, we calculate the excess noise after the input filter cavity (mode cleaner) and the dependence of the detector strain sensitivity on laser frequency and amplitude noise, radio frequency oscillator noise, and scattered-light phase noise. We find that noise on the radio frequency sidebands generally limits the detector's sensitivity.

Adaptive photodetectors: novel approach for vibration measurements

We report on a simple high-sensitivity interferometric technique of detecting vibrations and present characteristics of laser vibrometer using GaAs and SnS 2 adaptive photodetectors based on the effect of the non-steady-state photoelectromotive force. It enables efficient direct conversion of high-frequency phase modulation of speckle-like optical waves reflected from the vibrating object into an output electrical signal with concomitant setting of optimal operation point of the interferometer and suppression of amplitude laser noise. The results of measurements of small vibration amplitudes of the mirror and diffusely scattering objects are presented. Preliminary studies at 1.06 μm showed that it is possible to detect ultrasonic vibrations with the amplitude of 0.2 Å with a signal power of 20 mW and a bandwidth of 15.5 MHz. This optical phase-to-electrical signal converter is not sensitive to ambient vibrations, thermal drift, amplitude laser noise and is therefore appropriate for industrial applications.

Harmonic mode-locking using regenerative phase modulation

In this letter, we report stable operation of a 1-GHz passively modelocked fiber laser synchronized by regenerative phase modulation. The minimum pulsewidth obtained was 600 fs and the laser amplitude noise was typical of low-noise passively mode-locked fiber lasers.

Balanced heterodyne signal extraction in a postmodulated Sagnac interferometer at low frequency

We describe a balanced-heterodyne postmodulated Sagnac interferometer signal extraction method that is suitable for gravitational wave detection. The method is simple to implement by placement of a polarization-selective modulator after the beam splitter in the dark port of the interferometer. The postmodulated Sagnac interferometer retains its common path advantage and exhibits insensitivity to laser frequency noise below, at, and above the heterodyne frequency. Balanced detection reduces sensitivity to laser amplitude noise. In this scheme mirror displacement signals were rf demodulated and observed from 0.2 to 10kHz.

Polarization-based balanced heterodyne detection method in a Sagnac interferometer for precision phase measurement

We describe a balanced heterodyne detection method for a Sagnac interferometer that uses a polarization-dependent beam splitter. The signal and the local oscillator are orthogonally polarized components of a single laser beam, permitting the detection of the signal by subtraction of two photocurrents produced in appropriate polarization projections. Using this scheme, we experimentally demonstrate a phase measurement with a sensitivity of 9x10(-10) rad/ radicalHz. The measurement is robust in the presence of laser frequency noise, as a result of preserving the common-path nature of the Sagnac interferometer, and of laser-amplitude noise, as a result of balanced detection.

Semiclassical model of semiconductor laser noise and amplitude noise squeezing. II. Application to complex laser structures

For pt.I see ibid., vol.33, no.11, p.2097-2104 (1997). We present noise studies of distributed feedback (DFB) laser structures, where spatial hole burning (SHB) plays a key role performed using the model described in part I of this paper with particular emphasis on the influence of SHB, on the coupling coefficient /spl kappa/L, and on the laser facet reflectivities. These structures exhibit high amplitude noise and the possible noise reduction is strongly reduced compared to Fabry-Perot structures. Distributed Bragg reflector (DBR) lasers are better candidates even if their performances are also strictly determined by SHB and the loss in the Bragg reflector. Finally, limitations due to gain suppression are demonstrated for such complex lasers structures. We conclude on the optimum laser structure for amplitude squeezed states generation.

Semiclassical model of semiconductor laser noise and amplitude noise squeezing. I. Description and application to Fabry-Perot laser

A semiclassical model of semiconductor laser noise, based on the Green's function method, is used to derive analytical formulas for the amplitude and frequency noise spectra taking into account incoming vacuum fluctuations and noise due to internal loss. This formalism also takes into account phenomena such as gain suppression as well as spatial hole burning (SHB). The amplitude noise squeezing is studied for Fabry-Perot structures pointing out the influence of the laser structural parameters. A complete agreement with already existing quantum mechanical models is found. However, extension of the model to SHB induces limitations in the squeezing performances, which are very important for more complex structures as will be pointed out in detail, in Part II.

Phase sensitive detection of light reflected from a Fabry–Pérot interferometer

We present an analysis of the Fabry–Pérot response to a phase-modulated light in the reflection mode, by considering the general problem of the lock-in detection at the pth harmonics of the rf modulating frequency. Suitable frequency modulation conditions for servo-locking purposes are obtained and the values of modulation index which maximize the sensitivity for the first, third, and fifth harmonics are found. Moreover, we investigate the effects of the residual amplitude modulation introduced by the electro-optic frequency modulator, the presence of laser amplitude and frequency noise, and the dependence of the achievable closed-loop frequency fluctuation spectrum on the modulation index and detection noise.

In-process sensor for surface profile measurement applying a common-mode rejection technique

An optical non-contact profilometer is presented for super-smooth surfaces, which applies a so-called common-mode rejection technique. Environmental disturbances and laser amplitude noise, which commonly exist in similar instruments, are overcome in this system. The overall simplicity of the optics and electronics, the low cost of the components and the ease of alignment make this a convenient system to implement.

Monitoring bending motions of piezoceramic plates with homodyne interferometer based on adaptive photodetectors

We report on simple high-sensitivity interferometric technique of detecting vibrations of piezoceramic plates using adaptive photodetectors. It enables efficient direct conversion of high-frequency phase modulation of speckle-like optical wave reflected from vibrating plate into an output electrical signal with concomitant setting of optimal operation point of the interferometer and suppression of amplitude laser noise.

Noise characterization of a self-mode-locked Ti:sapphire laser

The phase and amplitude noise spectra of a self-mode-locked Ti:sapphire laser are measured and compared with the noise characteristics of a colliding-pulse mode-locked (CPM) dye laser. The timing jitter of the Ti:sapphire laser is found to be approximately half that of the CPM laser for jitter rates above 50 Hz. The amplitude noise of both lasers is comparable for frequencies of as much as several hundred kilohertz. However, between 0.5 and 5 MHz the noise spectrum of the Ti:sapphire laser is 126-134 dB below the carrier, while the CPM laser noise is limited by the pump laser at 115-131 dB below the carrier.

Low-noise operation modes of a passively mode-locked fiber laser

Optimum low-noise operation modes of a passively mode-locked fiber laser are described and characterized. The laser produces 60 fs bandwidth limited pulses with root mean square amplitude fluctuations of 0.4% in a frequency-band from 30 Hz to 100 kHz. It is shown that low-noise operation up to 5 dB below the amplitude noise of the pump laser is possible with overall negative cavity group-velocity dispersion and a large difference between the intracavity loss under free-running and mode-locked laser operation. Additional frequency stabilization reduces the timing jitter to 4 fs in a frequency band from 1 kHz to 100 kHz and 110 fs in the 30 Hz to 100 kHz band. >

Measuring vibration amplitudes in the picometer range using moving light gratings in photoconductive GaAs:Cr

We report the measurements of picometer vibration amplitudes produced by a piezo mirror in an interferometric setup, using a GaAs:Cr photoconductive device. The moving light fringes (gratings) induce in this new detector periodic photocurrents characterized by efficient suppression of the low-frequency drifts of the working point and low sensitivity to the amplitude laser noise. The optimum operation conditions of this system are also shown.