The quantum mechanical limits to the fundamental noise performance of semiconductor lasers are reviewed. Recent advances in pushing the laser noise below theses limits are then discussed with emphasis on pump suppression, electronic feedback, and correlation techniques such as optical feedback. It is found that narrow-linewidth semiconductor lasers with sub-shot- noise photon statistics are within the reach of current technology.

A frequency-doubled Nd:YAG laser has been stabilized to hyperfine transitions in molecular iodine near 532 nm via modulation transfer spectroscopy. This technique, together with the low noise of the source, yields excellent SNR (500 in a d kHz bandwidth); thus, an impressive frequency stability is achieved. The nearly systematic-free resonance signals obtained by modulation transfer spectroscopy give a correspondingly encouraging reproducibility, estimated to be about +/- 300 Hz. With two such stabilized lasers were found a pressure shift of only -1.3 kHz/Pa over the range 0.4-4.0 Pa and a power-dependent frequency shift of 2.1 kHz/mW. We have also measured the absolute frequency of the component a₁₀ in the transition R(56)32-0 using the D2 line in Rb at 780 nm and an iodine-stabilized 633 nm He-Ne laser as references. The measured frequency is 563 260 223.471 MHz +/- 40 kHz. In turn, the absolute frequency of the D2 line was measured via the frequency difference between the D2 line and the two-photon transition 5S_1/2 - 5D_5/2 at 778 nm in Rb. Thus we now have realized a pure frequency measurement of this interval and of the 532 nm frequency.

Widely tunable laser sources are desirable for many experiments and applications. The goal of this work is to develop sources of high frequency stability, low frequency noise, and tunability through the entire gain bandwidth of the diode laser without mode hops. Mode hops are discrete jumps in both amplitude and phase that are detrimental in many tunable laser applications. Tuning performance and analysis of grazing incidence external cavity lasers are emphasized in this paper. The external cavity diode laser studied in this work is an effective low noise platform for accessing a wide range of elements from the periodic table for atomic absorption experiments such as process control and monitoring. Frequency doubling can be used to reach many elements not accessible at the diodes fundamental wavelength range. Vapor deposition, wavelength division multiplexing, coherent communications, and fundamental research are additional applications for widely tunable external cavity diode lasers.

Our recent detection of sub-Doppler molecular saturated-absorption lines at 1.5 micrometers , using low power laser diodes, has opened the way to a precise frequency metrology at this wavelength. Our present goal is to establish a network of highly accurate reference frequencies in this wavelength band, and to provide the simplest way to generate them for a practical frequency metrology, which is of great interest for optical fiber communications, of high resolution spectroscopy. We present an overview of our efforts toward this goal, and we show that we can expect, in the near future, to be able to assign an absolute frequency value with a approximately 100 kHz precision for almost all the above-mentioned 1.5 micrometers frequency standards.

The absolute frequency measurement of each hyperfine component of the 5S_3/2 and 5S_5/2 levels in rubidium was done at ENS more than one year ago using Ti-Sa lasers. We built two devices based on diode lasers to study some metrological properties. We measure the frequency differences between hyperfine components of the 5S_5/2 level and we calculate the corresponding hyperfine constants. We also measure the frequency interval between the 5S_3/2 and 5S_5/2 levels using a Schottky diode. The measured stability in terms of Allan variance is 3*10^-13t^-1/2 up to 2000 s. The light shift is investigated and the difference between our two systems is 1.7 kHz. The repeatability of one system is better than 10^-12 and will allow the absolute frequency measurement at this level via the LPTF frequency synthesis chain.

The increase low-frequency amplitude noise on several extended cavity diode lasers was measured when frequency of phase lock servos were applied using the injection current as the feedback channel. The AM noise increase inside the FM servo bandwidth is approximately that expected from the suppression of frequency noise uncorrelated with the inherent amplitude noise of the laser.

Amplitude-squeezed light has been generated by semiconductor quantum well lasers in numerous optical configurations, at both room and cryogenic temperatures. Various intensity noise sources associated with nonideal laser operation, in particular mode partition and polarization-dependent noise, can introduce intensity fluctuations that limit or remove amplitude squeezing. We discuss amplitude noise reduction in low-temperature free-running and injection-locking laser configurations and room-temperature injection locking and external-cavity laser configurations. Drive-current modulation with a nonclassical SNR improvement is achieved with an injection-locked laser.

Amplitude-squeezed states are generated from a room-temperature semiconductor laser using a combination of pump suppression and dispersive optical feedback. The laser amplitude noise is found to be sensitive to extremely weak feedback levels, of the order of 10^-8 of the output power. a reduction of the noise from 2% below the standard quantum limit (SQL) under free-running conditions to 19% below the SQL under optimal feedback conditions is obtained. A single mode theory is presented but is found to be inadequate in explaining the measured dependence of the noise reduction on the feedback power. A multimode theory including asymmetrical cross-mode nonlinear gain is proposed to explain this discrepancy.

We discuss different approaches to the generation of bright amplitude-squeezed light using second-order nonlinear effects in optical cavities. A 0.2 mW beam at 1064 nm exhibiting a 4 dB squeezing has been generated using a phase-sensitive type-I parametric amplifier pumped by frequency-doubled Nd:YAG laser. In applying frequency-doubling processes to the generation of amplitude-squeezed light, the appearance of parasitic parametric oscillation must be considered. This effect has been observed in a singly-resonant frequency doubler. We also describe recent developments in optical cavity design for doubly-resonant frequency doubling. Finally, we show theoretically that phase-mismatched second-harmonic generation in an optical cavity can be employed to generate a strong effective Kerr nonlinearity. This system appears promising for achieving optical bistability and for generating squeezed light of high power.

We propose a new scheme to improve the sensitivity of the interferometric phase measurement. In the scheme and amplitude-squeezed state generated from a constant-current- driven semiconductor laser is employed instead of a squeezed vacuum state.

We have developed a sub-shot-noise diode laser system and used it to perform sub-shot-noise FM spectroscopy. By inducing FM sidebands on a direct-current modulated external-cavity diode laser and injection locking a slave laser, we have suppressed the residual AM in the master by 50 dB and achieved a noise level of 0.8 dB below the shot-noise level. We expect that such techniques will provide useful and practical tools for future precision measurements and novel atom-photon interaction experiments.

We focus on the noise characteristics of semiconductor lasers from a theoretical point of view. More precisely, we investigate the possibility of reducing amplitude noise in single-mode semiconductor lasers by forcing emission of sub-Poissonian light. Starting from a statistical laser model based on the photon density matrix equation (master equation) applied to a saturable medium, we have derived a fundamental condition for semiconductor lasers yielding to amplitude noise reduction. However, practical results reveal that sub-Poissonian light emission could only be achieved in the limit by specially designed semiconductor laser structures such as microcavity lasers. On the other hand, conventional bulk lasers must be disregarded as suitable sources for sub-Poissonian light emission.

The investigations of correlation between intensity noise of semiconductor laser and junction voltage noise were made. The laser was high-power quantum well separate confinement heterostructure (SCH) InGaAsP/GaAs laser with (lambda) equals 0.8 micrometers . The full negative correlation near the threshold between photon flux noise and junction voltage noise in constant-current regime was obtained. It was shown that at room temperature the laser exhibited constant-current regime due to base resistance. The theoretical treatment of correlation coefficient versus pump current was proposed and the role of optical losses has been revealed. The comparison of theoretical and experimental behavior near the threshold were made.

We report on the development of optical resonators operated at cryogenic temperature. a miniature monolithic quartz crystal ring resonator has been operated at liquid helium temperature with a finesse of 330 at the Nd:YAG wavelength 1064 nm. A 3-cm-long Fabry_Perot cavity with mirrors optically contacted to a hollow fused silica spacer has been used for the frequency stabilization of two diode-pumped Nd:YAG lasers. The cavity exhibited a finesses of 240,000 at liquid helium temperature. The root Allan variance of the beat signal of the two lasers locked to two transverse modes of the cryogenic optical resonator (CORE) was below 10 Hz for integration times up to 100 s. Requirements for reaching sub-Hz instability for long times are briefly discussed and it is pointed out that COREs have interesting applications in high-precision fundamental physics experiments.

A frequency chain, derived from the one used to measure the absolute frequency ((nu) $= 473 THz) of the He-Ne/I₂ optical standard, is currently being implemented in order to measure the frequency of a diode laser stabilized on the two-photon transition of rubidium vapor. The measurement scheme is based on the comparison of the frequency of this near-IR potential secondary standard to the 13th harmonic frequency of the R(12)-CO₂/OsO₄ LPTF secondary standard at (nu) equals 29.096 THz. Recent results on the frequency synthesis are reported, enabling the testing of long-term stability of this Rb-locked system with respect to the IR reference standard.

Ultrahigh resolution spectroscopy and metrology require very stable lasers with a high spectral purity. For spectroscopy with a resolution up to 1 kHz at 30 THz, the laser stabilization on a strong molecular absorption line detected in an external cell can provide a stability of a few Hz/mn and a linewidth of about 10 Hz. The development of a new stabilization scheme which acts separately on the short- and long-term stabilities is in progress. The stabilization on a peak of a high-finesses ULE Fabry-Perot cavity by using a piezoelectric transducer and an acousto-optic modulator should yield a laser linewidth of better than 1 Hz. Frequency locking on a molecular saturation line detected in transmission of another Fabry-Perot cavity can provide a long-term stability of a few Hertz on several hours. Such performances are required for spectroscopy with a resolution better than 100 Hz and for the realization of a new generation of frequency standards in the 10-micrometers spectral region based on a signal of a two-photon Ramsey fringes experiment.

A single-mode optical fiber is a convenient and efficient transmission medium for optical signal. However, the optical insertion phase written on the light field by the fiber is very sensitive to the surrounding environment, such as temperature or acoustic pressure. This phase-noise modulation tends to corrupt the original delta-looking Hz-level optical spectrum by broadening it toward the kilohertz domain. Here we describe a simple and effective technique for accurate cancellation of such induced phase noise, thus allowing fiber-based optical signal transmission in very demanding high-precision frequency-based applications where optical phase noise is critical the system is based on double-pass heterodyne measurement and digital phase division by two to obtain the correction signal for the phase compensating AOM. The underlying physical principle is the fact that an optical fiber path ordinarily possesses an excellent degree of linearity and reciprocity, such that two counterpropagating signals can experience the sam phase perturbations. Overall, the fiber's kilohertz level of broadening is reduced to sub-millihertz domain by our correction.

We have undertaken a research directed to the realization of frequency-stabilized lasers for multifrequency optical communications in the 375 THz, 229 THz, and 193 THz (0.8, 1.3, and 1.55 micrometers ) bands. In this paper, we present an overview of our latest results in the 1.55 micrometers band. We compare the performance of optical frequency references based on lasers frequency-locked to acetylene molecules and rubidium atoms. The absolute vales and the frequency stability improvements are discussed. We also present techniques to transfer those performance to multiple frequencies for multifrequency communication systems. We study the use of an absolutely calibrated multimode Fabry-Perot optical resonator with transmission peals set at exact multiples of 100 GHz. We also study the use of a calibrated wavemeter based on a sum-frequency surface emitting multilayered nonlinear crystal to allow the precise tuning at any frequency in the vicinity of an absolute optical frequency standard.

The relative optical frequency stabilities of three optical reference units at 1547.82 nm were examined. The reference units were located at different places in Berlin and connected via several km of optical fiber. The three optical signals were heterodyned to measure the frequency stabilities, which were better than 2*10$=-11) (2.5 KHz).

A waveguide type optical frequency comb generator was developed at 1.5 micrometers wavelength region by utilizing a waveguide type phase modulator. It was confirmed that the envelope of sideband spectrum had a width of 4.3 THz. A multiplex optical frequency comb generation system was assembled, and modulation sidebands were generated on a space as wide as 10 THz. A heterodyne signal between two OFCs whose central frequencies are separated as large as).4 THz was detected. By utilizing the signal, a frequency offset locked loop system was realized between the two OFCs.

We describe the operation of an optical frequency comb generator and its utility in measuring optical frequency differences in the terahertz range. The comb generator is a high-frequency resonant electro-optic modulator capable of generating hundreds of sidebands with a sideband spacing of 17 GHz and a span of at ;east 3 THz. The span is currently limited by the dispersion of the lithium niobate substrate. By detuning the modulation microwave frequency we have obtained asymmetric frequency spectra which effectively increases the span to 4.6 THz. we will describe three experiments that make use of the optical frequency comb to measure a difference frequency of 1.45 THz, to frequency lock two optical sources at a frequency separation of 1.4 THz, and finally to phase lock an optical parametric oscillator at a frequency difference of 0.665 THz.

We describe new requirements for a frequency reference in terrestrial line systems using wavelength division multiplexing and optical amplifiers, and evaluate leading atomic or molecular references for suitability.

A diode laser spectrometer in the visible range was developed. To achieve narrow linewidth and high power, a master-laser/slave system was employed. High-resolution spectroscopy of the ¹S₀-³P₁ transition of Ca was performed and optical Ramsey fringes were observed with a resolution of below 36 lHz using a thermal atomic beam.

A channel spacing frequency stabilization system for optical frequency-division-multiplexing communications is reported. Considering variable environmental condition and device aging effects, we have designed two loops in the system using a Fabry-perot interferometer as a frequency reference. One loop is a fine-tuning one, which is a time-division-multiplexing frequency stabilization scheme adjusting the driving currents of all the transmitter. The other loop is a rough-tuning one, which is a series of newly designed digital temperature controllers in which microprocessors and electrical oscillation circuits rather than Wheatstone bridge- circuits are used to detect the temperature error signal in order to reduce laser operating temperature dependence on the environmental conditions and there are RS-232 interfaces for communications with the first loop.

The effects of optical feedback from a phase-conjugate reflector on a semiconductor laser are considered in this paper. It is found that under these conditions, the linewidth of a single longitudinal mode diode laser can only be narrowed, never broadened. This is in contrast to feedback from a conventional dielectric mirror. For a multilongitudinal mode semiconductor laser the lasing envelope is found to be narrowed as well. The optical spectra for these cases is discussed and experimentally demonstrated.

When an optical signal that is corrupted by laser phase noise is transmitted through a chromatic dispersion limited optical fiber, the phase noise gets converted to intensity fluctuations. In this paper, the performance limitations of coherent optical systems in the presence of the fiber dispersion induced amplitude noise are described. Further, the power penalty incurred due to the presence of this noise with reference to ideal (dispersion-free) transmission is computed from the bit error rate expressions. The two-branch and three-branch ASK phase diversity receiver using squarers have been analyzed for various values of laser linewidth and chromatic dispersion parameter.

A dense WDM transmission system is promising for high-capacity fiber optic links. In a dense WDM system with a moderate number of channels, the total wavelength range can be within the discrete/continuous tuning range of multisection DBR lasers. Therefore, a laser array consisting of laser diodes with the same nominal wavelength can be used. In this paper, we propose a new frequency stabilization and channel identification technique for dense WDM transmission systems. The laser diodes on the transmitter side are frequency modulated by sinusoidal signals that are used as pilot tones for channel identification on the receiver side. Depending on the difference between the optical filter's central frequency and the received optical signal frequency, the filter output can be the sinusoidal signal itself or its second harmonic. We use a nonlinear electronic circuit and a phase-locked loop to regenerate the sinusoidal signal. This provides a reliable source for channel identification. Meanwhile, there is no need to modulate the optical filter central-frequency to lock it to the received signal since the received signal is already frequency-dithered and the sinusoidal signal is recovered on the receiver side. This eliminates the power penalty due to frequency-dither of the optical filter.