Single-mode lasers below 630 nm are still realized using complex laser systems. We present distributed Bragg reflector (DBR) ridge waveguide lasers (RWL) based on AlGaInP. When packaged into sealed TO-3 housings and cooled internally to about 0°C the DBR-RWL emit more than 50 mW at a wavelength of 626.0 nm into a nearly diffraction-limited single longitudinal mode with a spectral width below 1 MHz. These new monolithic diode lasers have the potential to drastically miniaturize existing set-ups e.g. for quantum information processing.
© 2013 Optical Society of America
The red spectral region between 600 nm and 630 nm as well as its higher harmonics are interesting for many spectroscopic applications, yet due to the unavailability of direct emitting single-mode diode lasers complex laser systems, such as ring-dye lasers  or frequency combined lasers  are widely employed. In principle, GaInP quantum well material offers gain in the wavelength region, however, actual laser emission is difficult due to the finite height of the barrier material . So far room temperature emission as low as 625 nm has been demonstrated using Fabry-Pérot broad area lasers . To push the emission wavelength to even lower wavelengths it was shown that the lasing wavelength in AlGaInP based laser diodes can be altered by cooling and pressurizing .
For single-mode operation the laser requires wavelength stabilization and lateral mode-filtering. While the latter can be achieved with ridge-waveguides (RW), the former is usually obtained by using gratings. It has already been demonstrated that gratings can be used to stabilize the wavelength of AlGaInP diodes over a large tuning range of 8 nm and that these external cavity diode lasers (ECDLs) can be miniaturized to a few centimeters [6, 7]. However, the active coupling of external gratings requires accurate alignment and is hence elaborate. Furthermore, it leaves the resonator to be susceptible to environmental disturbances such as air flow or mechanical shock. If only a single pre-selected wavelength is required, integrating the grating into the laser chip itself is the method of choice, since it alleviates these detrimental effects. Internal gratings in AlGaInP lasers have already been demonstrated in the wavelength region between 630 nm and 640 nm [8–10]. Here a single-mode operation of the laser with a linewidth as low as 1 MHz could be achieved . Lowering the wavelength below 630 nm could significantly simplify current experimental set-ups, e.g. for the cooling of CaF molecules  or of beryllium ions .
For the cooling of Be ions first experiments with GaInP gain material in external cavity configuration were published recently [14, 15]. These pose a significant reduction in size and complexity in comparison to dye laser setups. However, the whole ECDL has to be stabilized in temperature, in order to maintain a stable laser line. This leaves still a footprint of several tens of centimeters and further miniaturization is highly desirable for future applications beyond a laboratory environment.
We present in this paper distributed Bragg reflector (DBR) ridge waveguide lasers (RWL) emitting near 626.6 nm at room temperature. These DBR-RWLs can be tuned as low as 624.8 nm via the temperature within an encapsulated TO-3 package. This is to our knowledge the first presentation of a monolithic single-mode diode lasers at this wavelength, which have the potential to drastically reduce the size of current experiments and hence might enable many new applications.
2. Laser design and manufacturing
The laser structure bases on a tensile strained GaInP quantum well with a thickness of 15 nm (red in the inset in Fig. 1), which is embedded in AlGaInP based waveguide core layers (white). The cladding of the optical waveguide is asymmetrical with Si doped AlInP on the n-side (dark grey) and C doped AlGaAs on the p-side (light grey) . Pulsed measurements of uncoated broad area lasers of different length at 20°C resulted in an emission wavelength near threshold of λ = 634 nm, transparency current density of Jtr = 415 A/cm2, and internal efficiency of ηi = 0.75, a modal gain of Γg0 = 25 cm−1, and internal absorption of αi = 2.1 cm−1. Temperature dependent measurements of the threshold current from 15°C to 45°C of a laser chip with a resonator length of 1 mm resulted in a characteristic temperature coefficient T0 of 37 K. The vertical far field (fast axis) features only a fundamental mode with a full width at half maximum (FWHM) of 30.5°.
Based on experience of previously-manufactured DBRs from similar laser structures [11, 17], CAMFR (CAvity Modelling FRamework) simulations were performed [18, 19]. The DBR-grating is implemented by a periodic iteration of two slabs, simulating the etched and unetched regions, which feature different indices of refraction. The effective refractive index is calculated and used within the Bragg condition to determine the wavelength for maximum of the reflectivity. The grating period Λ of tenth order surface gratings was varied to simulate its effect on the emission wavelength of possible lasers (see blue dots in Fig. 1). An extrapolation (blue line) yielded a grating period of 960 nm to be suitable to obtain a maximum in reflectivity near 626 nm at 15°C. The red square is a measured wavelength of 626.6 nm (see also later Fig. 5).
The gratings were processed using wafer-stepper lithography and standard reactive ion etching (see inset in Fig. 2) as in [10, 11, 17]. In principle any arbitrary grating period can be processed . Here a period of 960 nm was chosen to demonstrate an emission near 626 nm. The same technology as for the gratings was used to define the ridge with a width of 5.0 µm and a refractive index step of approximately 2·10−3. Afterwards the grating section was covered with a dielectric and a metallization to allow soldering the chip with the p-side onto a sub-mount. Cleaved laser bars were passivated and coated to obtain reflectivities of Rf = 5% and Rr < 0.1%, at the front and rear facets, respectively. Single chips with a width of 400 µm were soldered onto a structured AlN submount and then mounted vertically onto a Peltier-element. The AlN submount also holds a temperature sensor with a distance of about 2 mm to the laser chip. This subassembly was soldered into a TO-3 package. The TO-3 package was hermetically sealed using a cap with a window that featured an anti-reflection coating for the 630 nm wavelength range with R < 0.5% (see Fig. 2).
The laser was mounted onto a heat sink (Newport 700-9-5.6) modified for accommodating the TO-3 package. The outside of the TO-3 was thus stabilized to a temperature of 15°C. The laser chip inside of the TO-3 could then be operated at temperatures between + 25°C and −30°C without the danger of condensation.
To assess the available spectral gain of the material the amplified spontaneous emission (ASE) from the front facet below threshold current was measured using a spectrometer with a resolution of 0.1 nm (Ocean Optics HR4000). The temperature was varied between + 25°C and −25°C in steps of 5 K. The spectra at + 25°C at 100 mA and −25°C at 30 mA are shown in Fig. 3. The spectra were fitted with Gaussian curves (not shown) resulting in center wavelengths of 634.1 nm and 625.9 nm. The center wavelengths for all temperatures are plotted as red dots in Fig. 6. A linear least squares fit yielded a temperature shift of the peak of 0.155 nm/K, which is similar to 0.14 nm/K found in  and the theoretical value of 0.13 nm/K obtained from linear interpolating the Varshni parameters of the binary components GaP and InP .
3. Laser Properties
The voltage-current and power-current characteristics are shown in Fig. 4 for the temperature range from + 15°C to −30°C. The forward voltage increases slightly by 0.05 V, which corresponds well with the widening of the bandgap due to the cooling by 45 K. The series resistance remains almost constant at about 1.9 Ω. Laser operation could be observed at currents below 140 mA up to 15°C. The output power increases drastically from 17 mW to more than 100 mW at the test current of 140 mA, when reducing the laser temperature from 15°C to −30°C. At the same time the threshold decreases from 120 mA to about 38 mA and the slope efficiency increases from 0.69 W/A to 1.02 W/A. Hence, an electro-optical conversion efficiency of 32% of the laser chip itself could be reached at −30°C. However, to maintain the cooling the internal Peltier element had a power uptake of about 3 W.
The spectral distribution of the laser emission was analyzed with a high-resolution spectrometer (LTB Demon) with a resolution limit of about 6 pm. It was found that the diodes showed single mode emission up to an output power of about 50 mW with side mode suppression ratios > 25 dB (see Fig. 5). Above 50 mW a second spectral peak appears approximately 120 pm below the main peak, which might be due to a secondary reflectivity peak of the DBR grating. The graph shows that the peak of the laser emission can be tuned over almost 2 nm from 626.6 nm at 15°C down to 624.8 nm at −30°C.
The peak positions of the laser emission are plotted as a function of temperature as black squares in Fig. 6. A least squares fit results in a slope of about 0.039 nm/K. In comparison to the peak position of the amplified spontaneous emission (red dots), the shift is significantly reduced. At + 15°C the distance between the peak of the ASE and the laser emission is large and reduces with the cooling. The peak positions meet at about −30°C, indicating this to be the optimum operation condition.
At + 15°C the detuning between the laser emission and the peak position of the ASE is as large as 5 nm. Hypothetically, if the same detuning still results in laser emission at −30°C, a simple DBR diode laser in a TO-3 could be envisioned to be lasing as low as 620 nm when implementing a grating period of about 952 nm.
While coarse tuning can be done with the temperature, the operation current allows fine tuning of the emission with a tuning rate current of about 0.74 pm/mA. This allows the measurement of the linewidth by tuning two lasers to the same wavelength and using a heterodyne measurement to detect the beat signal. Two laser diodes in TO-3 packages were mounted on separate holders (Newport 700-9-5.6) to allow individual temperature and current tuning using low-noise current sources (Newport-ILX LDC-3724C) and low noise filters (LNF-320). The LDC-3724 also operated the Peltier elements within the TO-3, while the Peltier coolers of the laser holders were operated using two temperature controllers (a Newport 3040 and a Thorlabs TED 350) to maintain a constant temperature of 15°C on the outside of the TO-3s. The backsides of the laser holders were water-cooled to maintain stable long-term operation. The light from each of the lasers was collimated using aspherical lenses with a focal lengths of 8 mm (Thorlabs C240-TME) then guided through optical isolators (Thorlabs IO-2D-633-VLP) and coupled into an optical fiber beam splitter (FOC BG03 2 × 2 SWC). One optical path also contained a half-wave plate to adjust the polarization direction. One output of the fiber coupler was connected to an optical spectrum analyzer (Advantest Q8384) to monitor the optical spectra, and the other output was mounted to a 12 GHz photodiode (Newport 1580-A) to measure the beat signal. To record the beat signal a spectrum analyzer (Rohde + Schwarz FSV) was used, with a resolution bandwidth of 100 kHz. The beat signal from the photodiode was also fed into a custom-built frequency-voltage converter and a PI controller, which generated a feedback signal to one of the low-noise current sources, to create a small current offset and thus maintain a constant frequency offset of about 54 MHz between the two lasers . This “weak” locking prevented the drift of the beat signal and allowed a long averaging of 1000 times to reduce the noise.
The averaged beat signal of two DBR-RWL at a current of 120 mA and an internal temperature of 0°C is shown in Fig. 7. At this operation point both lasers emitted about 50 mW at a wavelength of about 626.0 nm.
The beat signal has a FWHM of 1.43 MHz, giving a technical linewidth of about 1.0 MHz for each laser, assuming that both individual lasers contribute the same amount to the width of the beat. Least squares fitting of the curve with a Lorentzian and using a weighting proportional to the inverse of the square of the fitted values, gives an estimate of the intrinsic laser linewidth of about 320 kHz. This linewidth is significantly lower than e.g. the natural linewidth of 19.4 MHz of the 2S1/2 to 2P3/2 transition of 9Be+  and hence sufficient for laser Doppler cooling.
At the operation point of 50 mW the horizontal beam quality (slow axis) was measured using the method of the moving slit as in . Both intensity distributions, of the near field and far field, are shown in Fig. 8. Both near field and far field resemble Gaussian shapes. From the widths of the intensity distributions a beam quality factor of M2σ = 1.9 (M21/e2 = 1.7) was calculated. The fundamental mode content is estimated to about 80-90%, which should enable high efficient single-mode fiber coupling or second harmonic generation.
We developed DBR-RW lasers in a sealed TO-3 package to allow operation at temperatures as low as −30°C. It features single frequency emission up to 50 mW at wavelengths between 625 nm and 626 nm. At about 0°C the emission is close to 626 nm and exhibits a linewidth of about 1 MHz. These features enable easy-to-use laser sources in particular for laser cooling of Be + ions using resonant cavity second harmonic generation and in general for atomic and molecule spectroscopy in the wavelength range between 620 nm and 630 nm.
The authors would like to thank all colleagues at the FBH who made this work possible.
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