Diode Laser Spectra Research Articles

Optical and electrical degradation behavior of GaN-based UV-A laser diodes

The degradation mechanism of GaN-based ultraviolet (UV-A) laser diodes (LDs) is analyzed by studying the changes of their electrical and optical properties after aging process with the operation current below the threshold current (Ith). After aging treatment, Ith increases, the slope efficiency decreases, and the leakage current increases. In particular, the emission spectra of aged LD show significant broadening, with additional peaks on the shorter wavelength side. Both cathodoluminescence and deep level transient spectrum results indicate defects can develop around the active region of the LD, even when the operation current is below Ith. This leads to an increase in trap-assisted tunneling current and non-radiative recombination. The observed defects may result from the diffusion of water molecule in the environment from the cavity facet into the active region and formation of ON–CN and VGaON complex defects. These defects not only reduce the effective carrier concentration injected into the quantum well but also increase non-radiative recombination. These results will contribute to our understanding of the degradation mechanism of UV LDs and the preparation of long lifetime UV LDs.

Fabry-Pérot laser diode spectrum switching by controllable self-injection locking to a high-quality-factor optical microresonator

We propose a technique for automatic search and identification of possible generation regimes of a commercial multimode Fabry-Pérot laser diode self-injection locked to a high-quality-factor whispering gallery mode microresonator. Such a technique provides the potential for controllable switching between regimes with different wavelengths and with a specific number of lasing modes.

Emission Spectrum of an External Cavity Diode Laser under the Combined Action of Microwave and VHF Modulation of the Injection Current

Emission Spectrum of an External Cavity Diode Laser under the Combined Action of Microwave and VHF Modulation of the Injection Current

Infrared laser spectroscopy of the ν3-ν4 difference band and detection of the ν3 band of NO3

The infrared diode laser spectrum of the ν3-ν4 difference band of 14NO3 has been observed in the 690 cm−1 region. By fixing the lower state (v4 = 1) constants to the previously determined values, molecular constants in v3 = 1 have been determined for the first time : the vibrational energy E3 = 1054.13679(69) cm−1, the rotational constants B3 = 0.4562291(41), C3 = 0.2274567(53) cm−1, the first-order Coriolis coupling constant Cζ3 = 0.0032073(99) cm−1, and the spin–orbit interaction constant aeff = –0.03979(70) cm−1, with one standard deviation in parentheses. The ν3 fundamental band has been detected with weaker intensity than the ν3 + ν4 band by a factor of ~ 2000.

Fourier transform microwave spectroscopy of the SiCl+ ion

Fourier transform microwave spectra for the J = 1 ← 0 and 2 ← 1 rotational transitions of the SiCl+ ion were observed for two isotopologues (35Cl and 37Cl) in the ground and the first excited vibrational states of the ground 1Σ+ electronic state. Thanks to the high resolution of the FTMW spectrometer, hyperfine structures due to the quadrupole moment of the chlorine nucleus and the nuclear spin-rotation interaction were fully resolved. The observed FTMW spectra were combined with previously reported MMW and diode laser spectra in an analysis to determine the mass-independent Dunham coefficients Uk,l as well as a mass scaling parameter Δ01Cl=-0.856(30). The equilibrium bond length of SiCl+ determined is re=1.9439729(10) Å and the nuclear quadrupole coupling constant of Si35Cl+ is eQqe=-11.8788(23) MHz.

Diode lasers with front surface high-order distributed Bragg reflector

Control of spectral and spatial parameters of semiconductor lasers is a key element of their development and application. Surface integrated high-order distributed Bragg reflector (DBR) is the way to narrow diode laser spectrum preserving the conventional postgrowth process of laser diodes. On the other hand, high-order DBR has specific optical losses, which reduce optical output power. In this work we analyzed these specific optical losses and present design of laser resonator containing short front DBR, that allow to significantly reduce losses. With this approach, slope efficiency was greatly increased in contrast with conventional rear DBR laser diodes and 4 W optical output power with spectral width of less than 3 A was attained.

Narrowing of the emission spectra of high-power laser diodes with a volume Bragg grating recorded in photo-thermo-refractive glass

The spectral characteristics of a high-power semiconductor laser with an external cavity in which longitudinal-mode selection is implemented by means of a holographic volume Bragg grating recorded in photo-thermo-refractive glass are investigated. Mode selection leads to narrowing of the spectrum of the laser diode from an initial width of 6 nm to 5 A. In the narrowed spectrum, a fine structure representing a set of eigenmodes that correspond to the length of a laser chip is resolved. The number of these modes is limited by the width of the spectral selectivity curve of the grating. It is shown that insertion of the grating into the cavity leads to severalfold enhancement of the grating selectivity. It is demonstrated that the implemented external-cavity design makes it possible to smoothly tune the laser emission wavelength in a 10-nm range.

Millimeter-wave spectroscopy of the SiCl+ ion

The millimeter-wave spectrum of the SiCl+ ion in the ground and first excited vibrational states was observed for the two isotopic (35Cl and 37Cl) species. The ion was generated in a free-space absorption cell by a hollow cathode discharge of SiCl4 diluted with He and discriminated from neutral species by the magnetic field effect on the absorption lines. The observed millimeter-wave spectrum was combined with a previously reported diode laser spectrum in an analysis to determine mass-independent Dunham coefficients as well as the mass scaling parameters. The equilibrium bond length of SiCl+ determined is re=1.943978(2) Å.

On the vibrational assignment in the ground electronic state of NO3

The vibration–rotational structure of the ground A2′ state of the NO3 radical is re-considered, because Hirota (2015) presented an objection to the vibrational assignment of our previous papers (Kawaguchi et al., 2011, 2013; Fujimori et al., 2013). The present study examines the vibrational assignment in the following points, (1) laser induced fluorescence (LIF) spectra observed by Fukushima and Ishiwata in the 1050cm−1 region, (2) Fourier transform (FT) spectra of vibrational bands from the v4=1 state, and a diode laser spectrum of the 2ν4 band. These considerations support our previous assignments, that is, ν3+ν4=1492cm−1 and ν3=1055cm−1 for 14NO3.

The ro-vibrational analysis of the v4 fundamental band of CF3Br from jet-cooled diode laser and FTIR spectra in the 8.3-μm region

The v4 fundamental band of CF379Br and CF381Br, present in natural isotopic abundance, was investigated in the 8.3-μm region by high-resolution infrared spectroscopic techniques. Tuneable diode laser spectra were recorded in the ranges 1202.5–1205.0 cm−1, 1208.0–1210.1 cm−1 and 1212.5–1214.5 cm−1. The tuneable diode laser spectra were obtained at the reduced temperature of 200 K and in a free-jet expansion. The latter technique was used to reduce spectral congestion, achieving a rotational temperature of about 50 K, with a resolution up to 0.0008 cm−1. A Fourier transform infrared spectrum covering the entire spectral region of the v4 band, between 1190 and 1220 cm−1, was recorded at 298 K with a resolution of 0.004 cm−1. The experimental wavenumbers from the different spectroscopic techniques were combined to accomplish the complete ro-vibrational analysis of v4. In total, 4651 transitions were assigned to CF379Br, 4047 to CF381Br, with J″max = K″max =80; of these, 3171 for CF379Br and 2755 for CF381Br are from diode laser measurements. The data of each isotopologue were analysed using the model Hamiltonian for a degenerate vibrational state of a molecule of C3v symmetry. The v4 band of both the isotopologues resulted essentially unperturbed, but the Δl = Δk = ±2 l-resonance was found to be active within the v4 = 1 state. Precise values of the vibrational energy and of the ro-vibrational parameters of v4 = 1 for CF379Br and CF381Br were obtained. The bromine isotopic splitting amounts to 6.9 × 10−3 cm−1. In addition, the equilibrium geometry and the harmonic force field were calculated ab initio using the large-size basis set def2-QZVP in conjunction to the PBE0 functional.

Design and applications of discrete mode Fabry-Perot diode lasers

We describe how the multimode spectrum of a Fabry-Perot diode laser can be tailored using a non-periodic patterning of the cavity effective index. The cavity geometry is obtained from the solution of an inverse problem based on a perturbative calculation of the threshold gain of the longitudinal modes of the cavity. Experimental measurements are presented that demonstrate an all-optical memory element based on the injection locking bistability of a two-mode device. We also demonstrate passive harmonic mode-locking of a device designed to support a comb of six modes. Near-transform limited pulsed output with 2 ps pulse duration at 100 GHz repetition rate was obtained. Prospects for the extension of our approach to locking of larger numbers of modes over wider bandwidths are discussed. Similarities between the effective index profiles found in these devices and those of related devices and grating structures are also highlighted.

Continuous-wave cavity ring-down technique for accurate measurement of high reflectivity

A continuous-wave cavity ring-down (CW-CRD) technique employing a broadband diode laser is developed for high reflectivity measurement. The theory of square-wave modulated CW-CRD is presented. The spectrum of the broadband CW diode laser covers numerous free spectral ranges (FSRs) so that sufficient laser power is coupled into the cavity. Both amplitude and phase-shift of the first harmonic of the CRD signal, measured at an appropriate frequency range, are detected by a lock-in method and fitted to obtain the ring-down time and reflectivity. The measurements are repeated with five different cavity lengths and all the fitted reflectivities are in excellent agreement, indicating a high reliability of the CW-CRD technique. The reflectivity of the cavity mirror is determined statistically to be 99.70%, with an uncertainty of 0.01%.

Jet-cooled diode laser spectrum and FTIR integrated band intensities of CF3Br: rovibrational analysis of 2ν5 and ν2 + ν3 bands near 9 μm and cross-section measurements in the 450–2500 cm−1 region

The high resolution (up to 0.0007 cm−1) infrared diode laser spectrum of CF3Br (Halon 1301) with natural isotopic abundance has been investigated in the region 1090–1130 cm−1, characterized by the presence of the 2ν5 and ν2 + ν3 absorptions and several weaker hot bands. The rovibrational analysis of the data, obtained employing a slit-jet system (rotational temperature of about 50 K) together with those recorded at 200 K, led to a complete and accurate set of spectroscopic constants for the bands of both the bromine isotopologues. Integrated band intensities have been obtained for all the absorptions in the spectral range 450–2500 cm−1. The high resolution data and information concerning the cross-sections of this molecule can be useful for its remote sensing and for better modelling its contribution to global warming.

High Resolution FTIR and Diode Laser Supersonic Jet Spectroscopy of the N = 2 HF Stretching Polyad in (HF)2 and (HFDF): Hydrogen Bond Switching and Predissociation Dynamics

We report Fourier transform infrared (FTIR) and high resolution diode laser spectra (∼ 1MHz instrumental bandwidth) obtained in cooled absorption cells as well as in a supersonic jet expansion for the N = 2 polyad region of the HF-stretching vibrations of (HF)2, HFDF and DFHF. Three vibrational transitions have been observed for (HF)2 and two for both monodeuterated isotopomers. For (HF)2 we have identified and analysed the observed transitions of the polyad member 22 of the type Δ K a = 0 and Δ K a = ± 1 up to rotational sublevel Δ K a = 3. Band centers as well as rotational constants of all four K a states have been determined. The tunneling splittings due to hydrogen bond switching for these four K a states have been investigated, with the Δ K a = 0 up to Δ K a = 2 sublevels having tunneling symmetry Γ vt = A + for the lower tunneling states, and switching periods ranging from 158ps for K a = 0 to 1.35ns for K a = 2. A tunneling level inversion is found at Δ K a = 3, leading to a symmetry Γ vt = B + for the lower tunneling state of this K a-sublevel. The vibrational assignment of the measured spectra of (HF)2 was established by comparison with the monodeuterated isotopomers HFDF and DFHF. For HFDF we have identified and analysed five subbands between 7600cm-1 and 7730cm-1. We have determined the spectroscopic constants of the rotational levels Δ K a = 0 and Δ K a = 1 for the vibrationally excited state and of the levels of Δ K a = 1 and Δ K a = 2 of the ground state, the latter from combination differences. From the measurements in a supersonic jet expansion we determined the predissociation line width of the N = 22, K a = 1 to be about 120MHz for the Γ vt = A + tunneling state of (HF)2 and about 90MHz for Γ vt = B +. For the Δ K a = 0 level of N = 22 we obtained predissociation line widths ranging around 100MHz, similar to those of the Δ K a = 1 level. In the case of HFDF, the predissociation line width of Δ K a = 1 is about 80MHz. Predissociation lifetimes for these levels with the unbonded HF stretching excited thus are in the range of about 1 to 2ns. The predissociation width in the N = 21 level is uncertain by about a factor three with lg(Δν/MHz) = (3 ± 0.5) and in N = 23 it is about 600MHz corresponding to rounded lifetimes of 0.1ns and 0.3ns when the bonded HF stretching is excited thereby demonstrating strongly mode selective predissociation rates in the N = 2 polyad. Under thermal equilibrium conditions we derived the pressure broadening coefficient for (HF)2 (γ = (6 ± 1) × 10-4cm-1/mbar in the wavenumber range between 7713cm-1 and 7721cm-1 for total gas pressures between 10 and 60mbar, all values as full widths half maximum). For absolute frequency calibrations we have remeasured the first overtone transitions of the monomer HF with much improved precision between P(5) (7515.80151cm-1) and R(7) (7966.22188cm-1).

Diode laser spectra and rovibrational analysis of the ν 4 fundamental of

The ν4 fundamental band of (ν0 = 1068.517 cm−1), corresponding to the C–F stretching mode, has been measured at high resolution (0.002 cm−1) with a tunable diode laser spectrometer at a temperature of 240 K. The rotational structure of the a- and b-type components of the hybrid band has been extensively interpreted achieving high values of J and K a quantum numbers. Ground state constants up to sextic terms have been provided for the first time by using the combinations method from 11330 assigned transitions. The spectrum has been analysed using the A-reduced Watson's Hamiltonian in the Ir representation. From spectral simulations the dipole moment ratio |Δμ a/Δμ b| of the band has been estimated to be 1.8 ± 0.4. Additionally, simulations and comparisons with spectra of the natural isotopic sample are presented.

Design and evaluation of a visible-to-near-infrared electronic slitless spectrograph

Most commercial diffraction grating based spectroscopic devices employ entrance slit and optics based on the conventional Czerny–Turner (CT) configuration. A study was undertaken to design and develop a novel lab-scale electronic slitless spectrograph (ESS). The developed ESS has a tuneable spectral detectable range from approximately 450–1100 nm and a scanning speed of 0.7 s per spectrum. It works under reflectance mode with external illumination. System design employed a fibre optic waveguide as the probe, several focusing and collimating lenses, a plane reflection diffraction grating and a linear array charge-coupled device (CCD) detector. A method using a single-mode diode laser to calibrate the wavelength and pixel relationship was applied to obtain a least-square regression equation. An argon lamp was used to validate the calibrated wavelength equation and a maximum wavelength error of 1.95 nm was reported. A procedure to estimate the instrumental optical band-pass using laser spectra with a limited number of sampling points was examined. An optical band-pass expressed as full-width-at-half-maximum (FWHM) of 6.71 nm was estimated using the proposed method on the spectrum of the single-mode diode laser. The test results suggest that the developed ESS system has the potential to serve as a rugged and cost-effective spectroscopic device for routine inspection. The prototype consists of no moving parts and the complete assembly measures 30 cm × 25 cm × 15 cm, which can be greatly reduced upon further development.

Application of α–β–γ filtering to real-time atmosphere methane concentration measurement

A real time α–β–γ filtering technique is applied to the monitoring of atmosphere CH4 based on a tunable diode laser spectrum system operating at 1.654 μm. This technique is developed for improving the sensitivity and precision of CH4 concentration measurement with slow concentration change. The effectiveness of this technique is evaluated by performing CH4 concentration measurement and using it to monitor the varying methane level in the atmosphere. It was proved that signal noise ratio enhancement factor is 4.25. The comparison between this filter and moving average is also included in this article. It indicates the advantage of the α–β–γ real time filter.

External cavity tunable diode laser spectrum of the ν1+ ν3 N–H stretching combination band of 15NH 3

The infrared spectrum of the 15NH 3 isotopic species of ammonia has been investigated in the 6370–6800 cm −1 region of the ν 1+ ν 3 N–H stretching combination band with an external cavity tunable diode laser (ECTDL) spectrometer. A group of the stronger absorption lines form a pattern that is systematically shifted downwards by 12.2 cm −1 from a closely similar pattern of known transitions in the ν 1+ ν 3 band of normal 14NH 3, allowing immediate line assignments. This isotopic shift is almost exactly compensated by changes in rotational energy such that the r R[( J+1) K+1 ] and p P[( J−1) K−1 ] transitions of 15NH 3 lie very close to the 14NH 3 r R[ J K ] and p P[ J K ] transitions, respectively, giving a convenient comparison of relative intensity. From spectra of a 50:50 mixture of the two species, the absorption strengths appear comparable, with a slightly greater transition moment for 15NH 3.

Jet-cooled diode laser spectra of CF3Br in the 9.2 μm region and rovibrational analysis of symmetric CF3stretching mode

Pulsed slit-jet high resolution (up to 0.0009 cm(-1) FWHM) infrared diode laser spectra of CF(3)Br, with natural isotopic abundance, were obtained in the region around 9.2 microm at the rotational temperature of about 50 K. In addition, diode laser spectra at reduced temperature (200 K) were recorded. We present here the rovibrational analysis of the nu(1) fundamental in the range 1075-1090 cm(-1). The fine structure of many P(J) and R(J) clusters has been well resolved for the first time. The assignment of rovibrational transitions has been accomplished up to K = 27, J = 63 for CFBr and K = 33, J = 62 for CFBr. A total of 636 (CFBr) and 880 (CFBr) lines were used in the final fit and a very accurate set of molecular constants, including the quartic centrifugal distortion coefficients for the v(1) = 1 state of both the bromine isotopologues, was obtained. In addition, spectral features belonging to the nu(1) + nu(6)-nu(6) hot band were unambiguously identified and a set of spectroscopic parameters were determined.

Asymmetric Russian-doll model for semiconductor lasers

We demonstrate that the asymmetry of a high-power laser diode spectrum with its typical blue tail is related to frequency-dependent diffraction losses. A two-dimensional differential near-field analysis shows that the red longitudinal modes exhibit more self-focusing than the blue ones, reducing their diffraction losses, leading to as much as 20% variation of measured filament sizes in the plane parallel to the junction. The topological analysis of the far-field allows us to isolate the higher losses experienced by the blue modes, in agreement with a wavelength-dependent Kerr-lens model.