Showing papers on "Laser linewidth published in 1986"

Theory of spontaneous emission noise in open resonators and its application to lasers and optical amplifiers

Abstract: A theory of spontaneous emission noise is presented based on classical electromagnetic theory. Unlike conventional theories of laser noise, this presentation is valid for open resonators. A local Langevin force is added to the wave equation to account for spontaneous emission. A general expression is found relating the diffusion coefficient of this force to the imaginary part of the dielectric function. The fields of lasers and amplifiers are found by solving the wave equation by the Green's function method. The lasing mode is a resonant state associated with a pole in Green's function. In this way, noise in lasers and amplifiers is treated by a unified approach that is valid for either gain guiding or index guiding. The Langevin rate equations for the laser are derived. The theory is illustrated with applications to traveling wave and Fabry-Perot amplifiers and Fabry-Perot lasers. Several new results are found: optical amplifier noise increases inversely with quantum efficiency; spontaneous emission into the lasing mode is enhanced in lasers with low facet reflectivities; and the linewidth of a Fabry-Perot laser with a passive section decreases as the square of the fraction of the cavity optical length that is active.

Linewidth determination from self-heterodyne measurements with subcoherence delay times

Abstract: The behavior of the power spectrum of an external cavity semiconductor laser has been studied using a delayed self-heterodyne interferometric technique that uses delay times less than the laser's coherence time. Experimental results show that the resulting power spectrum is consistent with the theoretical model. However, there is evidence that additional frequency fluctuations are present that cause the delta function portion of the power spectrum to have a finite width.

Phase noise in semiconductor lasers

Abstract: The subject of phase noise in semiconductor lasers is reviewed. The description of noise in lasers and those aspects of phase noise that are relevant to optical communications are emphasized. The topics covered include: Langevin forces; laser linewidth above threshold and below threshold; line structure due to relaxation oscillations; phase fluctuations; line narrowing by a passive cavity section and by external feedback; coherence collapse due to optical feedback; the shot noise limits of several schemes of coherent optical communication, and the linewidth required to approach these ideal limits.

Balanced phase-locked loops for optical homodyne receivers: Performance analysis, design considerations, and laser linewidth requirements

Abstract: Balanced phase-locked loops for optical homodyne receivers are investigated. When a balanced loop is employed in a communications system, a part of the transmitter power must be used for unmodulated residual carrier transmission. This leads to a power penalty. In addition, the performance of the balanced loops is affected by the laser phase noise, by the shot noise, and by the crosstalk between the data-detection- and phase-lock-branches of the receiver. The impact of these interferences is minimized if the loop bandwidth B is optimized. The value of B opt and the corresponding optimum loop performance are evaluated in this paper. Further, the maximum permissible laser linewidth \delta u is evaluated and found to be 5.9 \times 10^{-6} times R b , where R b (bit/s) is the system bit rate. This number corresponds to BER = 10^{-10} and power penalty of 1 dB (0.5 dB due to residual carrier transmission, and 0.5 dB due to imperfect carrier phase recovery). For comparison, decision-driven phase-locked loops require only \delta u = 3.1 \times 10^{-4}. R_{b} . Thus, balanced loops impose more stringent requirements on the laser linewidth than decision-driven loops, but have the advantage of simpler implementation. An important additional advantage of balanced loops is their capability to suppress the excess intensity noise of semiconductor lasers.

Noise analysis of injection-locked semiconductor injection lasers

Abstract: The noise of injection-locked semiconductor lasers is analyzed by rate equations including the spontaneous emission noise. The side mode suppression and the relative intensity noise (RIN) of the locked laser (slave laser) are given for different wavelengths detuning between the master and slave laser and for different linewidth enhancement factors α. For large α, locking is difficult to achieve, whereas extremely low noise may be obtained for injection-locked lasers with a low linewidth enhancement factor.

Nonlinear dynamics and spectral behavior for an external cavity laser

Abstract: The influence of nonlinear dynamics on the linewidth, spectral behavior, and stability properties for a semiconductor laser in an external cavity is examined by numerical simulation of the noise-driven rate equations. Experimental results on the feedback-induced linewidth reduction are presented. The saturation of linewidth reduction and the sudden increase in linewidth which is observed at high feedback levels are shown to be contained in the theoretical model. The state with strongly increased linewidth has the signature of a chaotic state. We present examples of the calculated time evolution of the complex field amplitude and the associated FM noise spectra and field power spectra. The examples illustrate the transition to the chaotic state and focus on the spectral characteristics of the state.

Investigation of InGaAs-InP quantum wells by optical spectroscopy

Abstract: A detailed study of the optical properties of InGaAs-InP single quantum wells (QWS) grown by atmospheric-pressure metal-organic chemical vapour deposition is described. Photoluminescence (PL), photoluminescence excitation (PLE), photoconductivity (PC) and electroreflectance (ER) are employed to study both undoped and modulation-doped quantum wells. The role of extrinsic processes in determining the low-temperature PL spectra is demonstrated from the variation of peak position and linewidth with temperature. The best PL linewidth obtained for a 150 AA well is 5.3 meV, fairly close to the limit imposed by alloy fluctuations in the InGaAs. The role of free carriers in the undoped QWS in determining the energy threshold for optical absorption is demonstrated from a comparison of PLE and PC spectra. Lineshape fitting of the PL spectra is described, and it is deduced that at 160K recombination processes in both doped and undoped QWS proceed with wavevector conservation, although at lower temperatures highly anomalous lineshapes are found in modulation-doped samples. The observation of a threshold in PC spectra under forward bias is interpreted as a transition from the valence-band well to the top of the conduction well. The ratio of the conduction- to valence-band discontinuities is deduced to be approximately 0.4:0.6.

Phase noise and linewidth in an InGaAsP DFB laser

Abstract: Measurements and calculations of interferometrically demodulated phase noise in an InGaAsP DFB laser are reported. The results led to a novel method of laser linewidth measurement. The effect of this noise on a DPSK coherent system is considered.

Temperature dependence of the Raman linewidth and line shift for the Q(1) and Q(0) transitions in normal and para- H 2

Abstract: Using stimulated Raman gain spectroscopy, we have measured the temperature and density dependence of the Raman linewidth and line shift for the Q(1) and Q(0) vibrational transitions in ${\mathrm{H}}_{2}$ covering the range 77--480 K and 1--20 amagats, and as a function of ortho to para ratio. We conclude that the dominant contribution to the Q(1) linewidth comes from vibrational dephasing collisions, an effect not included in current theories. The line-shift coefficient has a much larger temperature variation than the linewidth coefficient, with the ratio of line shift to linewidth reaching a maximum value of 7.3 at 81 K. These results are important for applications requiring the tuning of the Stokes frequency generated by the stimulated Raman process.

Performance analysis and laser linewidth requirements for optical PSK heterodyne communications systems

Abstract: An optical PSK heterodyne communications receiver is investigated. The receiver is based on the decision-directed phase-locked loop. The performance of the phase-locked loop subsystem is analyzed taking into account both shot noise and laser phase noise. It is shown that for reliable phase locking (rms phase error less than 10°), heterodyne second-order loops require at least 6771 electrons/s per volt every hertz of the laser linewidth. This number corresponds to the limit when the loop dumping factor η is infinitely large; if \eta = 0.7 , then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency and \lambda = 1.5 \mu m, the above quoted numberers give 0.9 pW/ kHz for \eta \rightarrow \infty and 1.35 pW/kHz for u = 0.7 . The loop bandwidth required is also evaluated and found to be 155 \Delta u , where \Delta u is the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be 2.26 \cdot 10^{-3} R_{b} where R b is the system bit rate. For R_{b}=100 Mbit/s, this leads to \Delta u kHz. Such and better linewidths have been demonstrated with laboratory external cavity lasers. For comparison, ASK and FSK heterodyne receivers are much more tolerant to phase noise,-they can tolerate \Delta u up to 0.09 R b . At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth ( \Delta u ).

External-cavity semiconductor laser with 15 nm continuous tuning range

Abstract: A 1.26 μm optical amplifier with a facet modal reflectivity below 0.0001 has been used in a 58 mm-long grating external cavity. The lasing wavelength has been continuously tuned without mode hopping over a range of 15 nm by combined translation-rotation of the diffraction grating. A linewidth of 20 kHz has been derived from heterodyne beat frequency measurements between two tunable external-cavity lasers.

A review of stimulated Brillouin scattering excited with a broad-band pump laser

Abstract: Stimulated Brillouin scattering (SBS) excited by a pump laser with a bandwidth large compared to the Brillouin linewidth is reviewed. SBS amplifier results show that either for low gain or for short pulses, the broad-band SBS gain is substantially lower than the steady-state gain. In the high gain limit, however, the fast phase fluctuations that are typical of a broad-band pump have a small effect on amplifier gain. Results for SBS that grows from noise show that fast pump phase fluctuations affect the gain only under two conditions. The first is when the coherence length of the pump is shorter than the gain length in the SBS medium. The second is when the pump is made up of two or more temporal modes separated by more than the SBS linewidth and aberrated with independent spatial profiles. In this case, the quality of wavefront inversion may be badly degraded as well. Areas for further work, including competing nonlinearities, are discussed.

Theoretical analysis of heterodyne optical receivers for transmission systems using (semiconductor) lasers with nonnegligible linewidth

Abstract: We present a general theoretical model of receivers for coherent optical communication systems where transmitters and local oscillators having nonzero linewidth are used. Key issues in the model are the concept of single realization measurements of a stochastic intermediate frequency, and development of the probability density function for this stochastic process. Analytical results are derived for heterodyne ASK and dual filter FSK receivers and include the shot-noise limit, the asymptotic error-probability limits in ASK and FSK receivers, the influence of the IF on receiver noise, and the effective local oscillator strength. Detailed numerical results for typical p-i-n-FET wide-band receivers illustrate the influence on receiver sensitivity of IF filter bandwidth and relative threshold setting in ASK systems and of modulation index and IF filter bandwidth in FSK systems. A receiver sensitivity penalty for nonzero linewidth is found to be, for IF linewidths of 0.1 to 0.3 of the bit-rate, 3 to 9 dB in optimum ASK receivers, and 2 to 8 dB in optimum FSK receivers. Thus DFB lasers of linewidth 5 to 20 MHz could be used without external cavities in simple systems with near-ideal performance, which could find application wherever the great multiplexing advantage of coherent systems is a prime advantage. We present some guidelines for system design based on the results of this work.

Ultrahigh finesse fiber Fabry-Perot interferometers

Abstract: Fiber Fabry-Perots have been built with a finesse of 500 and throughput of 30-40 percent. The devices are piezoelectrically scanned and operate at 1.52μm, and have free spectral ranges of 1000, 500, 333, and 250 MHz, with linewidth resolution as small as 0.5 MHz. The sources of loss which limit the attainable finesse have been identified and analyzed.

Theory of mode specific vibrational predissociation: The HF dimer

Abstract: Quantum mechanical close coupling calculations on (HF)2 vibrational predissociation are presented. The model considers vibrational excitation of the proton donor monomer. The other momomer is frozen at its equilibrium position. The linewidth (lifetime) and final state rotational distribution of the initially vibrationally excited HF were calculated using the artificial channel method. The potential surface was taken from Cournoyer and Jorgensen. The calculated linewidth is in good agreement with recent experimental measurements. This result is drastically different from what is obtained in the collinear model, merely reflecting the important influence of the rotational–vibrational coupling in the highly anisotropic systems. The final rotational distribution of the initially excited HF monomer is highly inverted. The possibility of a rotational in conjunction with a conventional vibrational HF laser is raised.

Cancellation of fiber loss by semiconductor laser pumped Brillouin amplification at 1.5 μm

Abstract: The propagation loss is canceled, and 5 dB of net gain is obtained by Brillouin amplification in a 37.5‐km‐long single‐mode optical fiber. A narrow linewidth external cavity semiconductor laser is used as the pump source and a gain of 4.3 dB/mW pump power is measured. The Brillouin linewidth is measured to be 150 MHz in contrast to previous measurements of ∼20 MHz. The Brillouin Stokes shift was 11.3 GHz at 1.5 μm wavelength.

Effect of laser mode structure on stimulated Brillouin scattering

Abstract: The gain and reflectivity of the stimulated Brillouin scattering (SBS) process used as a Stokes-wave generator are shown theoretically to be independent of the mode structure of the pump laser provided that the pump-laser mode spacing exceeds the Brillouin linewidth and that the laser coherence length exceeds the characteristic gain length of the SBS process. Under the same set of conditions, the gain of an SBS amplifier is found to depend on the degree of correlation between the laser and Stokes fields. However, due to nonlinear coupling, these two fields become correlated within several characteristic gain lengths, and the subsequent propagation of the two fields is governed by the same set of equations that apply for the case of a single-mode pump laser. These theoretical predictions are tested experimentally for an SBS generator using acetone, carbon disulfide, and methanol as the Brillouin-active media, and the results are in full agreement with the theoretical predictions.

Quantum mechanical theory of linewidths of localized radiative transitions in semiconductor alloys

Abstract: A quantum mechanical formalism to calculate the effect of compositional disorder on the linewidth of localized radiative transitions in semiconducting alloys is developed. A variational‐statistical approach is used to calculate the line broadening. The general theory is applied to the specific case of excitonic transitions in which the exciton is in the ground state. The linewidth as a function of alloy composition is derived and the results thus obtained are compared with those of previous theories. For illustration, the calculations are done for GaAlAs and the results are compared with the available low‐temperature photoluminescence data.

Fibre Bragg reflector for mode selection and line-narrowing of injection lasers

Abstract: A surface-relief grating on a side-polished single-mode fibre acts as an external Bragg reflector for standard, multilongitudinal-mode 1.3 μm injection lasers. With 2 m fibre length, single-mode, 60 kHz linewidth operation at 1 mW is obtained.

Narrow linewidth, single frequency semiconductor laser with a phase conjugate external cavity mirror

Abstract: We measure the spectral characteristics of an external cavity semiconductor laser which uses a phase conjugate mirror for its external reflection. This device has significant advantages over the conventional external cavity system owing to the self-aligning nature of the phase conjugate mirror. The fiber delay line self-heterodyne technique is used to measure the fundamental linewidth for single mode operation of this device. It shows the linewidth to be at least as narrow as the instrumental resolution of 100 kHz.

Chemical beam epitaxial growth of extremely high quality InGaAs on InP

Abstract: Full widths at half‐maximum intensity of the (004) Bragg reflecton peak as small as 24 arcs are obtained from InGaAs epilayers 4–6 μm thick. Such linewidth is the narrowest reported thus far for an InGaAs epilayer grown by any vapor phase technique reported in literature. Such extreme compositon uniformity is also supported by results from Auger depth profiles and 2 K photoluminescence measurements. Very intense efficient luminescence peaks due to excitonic transitions with linewidths (FWHM) as narrow as 1.2 meV are obtained. This again represents the narrowest linewidth ever reported for InGaAs grown by any technique. In fact, such a linewidth represents the narrowest linewidth ever measured for any alloy semiconductor. Further, the photoluminescence spectra reveal the donor‐to‐acceptor pair recombination is nearly absent. This indicates that the InGaAs is of very high purity. Hall measurements of 2–5‐μm‐thick epilayers grown directly on InP substrates have mobilities of 10 000–12 000 and 40 000–57 000 c...

Semiconductor laser devices

Abstract: It is possible to narrow the emission spectrum linewidth of a semiconductor laser device by coupling optically an external resonator with one end surface of the semiconductor laser. However, the structure of the external resonator should match the phase of the light emitted by the laser. Heretofore, this matching has been effected by adjusting the length of the external resonator, and hence productivity and reproducibility have not been good. According to this invention, characteristics of the external resonator can be adjusted electrically to be matched with the phase of the emitted light owing to the fact that the external resonator is made of a material, whose refractive index can be varied electrically. Therefore the semiconductor laser device according to this invention is very practical.

A statistical analysis of ultraviolet, x-ray, and charged-particle lithographies

Abstract: We analyze photo, x‐ray, e‐beam, and ion‐beam lithographies from a statistical point of view in order to relate linewidth control to the contrast provided by the exposure technique, the resist contrast, and the resist sensitivity. Assuming a linewidth control of 20% at a minimum linewidth of 0.5 μm, we compare UV and x‐ray lithographies and find that the former is practical only with very high contrast resists (δN/N<0.16). We derive a simple expression for the minimum number of photons or charged particles required per pixel, and compare the pixel‐transfer rates of various lithographies. We find that beyond UV photolithography, synchrotron and pulsed‐plasma x‐ray sources offer the highest pixel‐transfer rates and appear most attractive for submicron lithography.

Theory of spectral linewidth of external cavity semiconductor lasers

Abstract: A new formula of the spectral linewidth of external cavity semiconductor lasers is proposed, with which linewidth narrowing with the optical feedback is discussed. It is shown that in the limit of large external cavity length, the linewidth caused by the phase diffusion due to spontaneously emitted photons becomes dominant and the linewidth decreases proportionally to the inverse of the external cavity length.

Methods of Laser Spectroscopy

Abstract: This book presents information on the following topics: the one-atom maser and cavity quantum electrodynamics; Rydberg atoms and radiation; investigation of nonthermal population distributions with femtosecond optical pulses; intra- and intermolecular energy transfer of large molecules in solution after picosecond excitation; new techniques of time-resolved infrared and Raman spectroscopy using ultrashort laser pulses; spectral linewidth of semiconductor lasers; the hydrogen atom in a new light; laser frequency division and stabilization; modified optical Bloch equations for solids; CARS spectroscopy of transient species; off resonant laser induced ring emission; UV laser ionization spectroscopy and ion photochemistry; laser spectroscopy of proton-transfer in microsolvent clusters; recent advances in intramolecular electronic energy transfer; and photoionization and dissociation of the H/sub 2/ molecule near the ionization threshold.

Crosstalk in optical amplifiers for two-channel transmission

Abstract: Optical amplifiers for two-channel transmission are investigated. Intensity modulation in one channel causes crosstalk in the other channel via the change of the complex refractive index in the amplifier. The amount of crosstalk is connected with the linewidth enhancement factor α.

Polarized anisotropic photoluminescence of laser‐related transitions in YAlO3:Nd and YAlO3:Er and line broadening by resonant lattice phonons

Abstract: Polarized anisotropic photoluminescence spectra are presented for the 4F−43/2 I11/2 transitions of YA1O3:Nd3+ and for the 4S3/2–4I9/2 transitions of YAlO3:Er3+. It is shown that differences in the individual linewidth are caused by different degrees of resonance between lattice phonons and sublevel spacings. This model is also successfully applied to Y3Al5O12:Nd.

Temperature of a laser-cooled trapped three-level ion

Abstract: We develop an expression for the final temperature achieved by laser cooling of a trapped three-level ion with the Λ configuration of states by employing an expansion in the photon momenta and degenerate perturbation theory. The special cases of low intensities or large detunings for the intermediate level are studied analytically. A cooling mechanism based on two-photon transitions with a simultaneous reorganization of the center-of-mass motion is isolated. The limit of the temperature is set either by the two-photon linewidth or by optical pumping between the long-lived states of the three-level system and may be extremely low.

Costas loop analysis for coherent optical receivers

Abstract: The Costas loop is analysed taking both shot and laser phase noise sources into account and it is shown that to avoid large performance penalties beat linewidth to bit-rate ratios smaller than 0.05% and 0.5% are needed for PSK homodyne and heterodyne systems, respectively.

Infrared photodissociation of benzene dimers in the 1000 cm−1 frequency region

Abstract: The infrared photodissociation of benzene dimers has been investigated in the frequency region of the CO2 laser. A single absorption peak is observed with a maximum at 1038 cm−1, and a full width at half‐maximum of about 2 cm−1. The laser fluence dependence of the dissociation yield and two‐laser hole burning experiments both indicate that the linewidth is mostly homogeneous. The benzene monomer fragments from the photodissociation emerge with an isotropic angle distribution. A relatively large fraction of the available energy goes into product translation, and the remainder into rotation.