Quantum sensors, such as optical atomic clocks, are increasingly used in out-of-the-lab environments. This development necessitates a high degree of integration and robustness across all components of these systems. In this context, we present the status of our efforts to develop and realize compact laser distribution modules for the implementation of low-SWaP (size, weight, and power) strontium-based optical clocks. These modules are designed to control and manipulate the properties of laser light, such as frequency, phase or intensity, prior to the light's interaction with the atoms of the clock. By integrating miniaturized free-space acousto- and electro-optic modulators into these modules, they enable applications such as laser power or fiber path length stabilization, as well as frequency-modulation spectroscopy. In this paper, we present a distribution module designed for 461 nm that was assembled using our hybrid micro-integration technology and will be integrated into a compact optical clock demonstrator. Additionally, we provide an outlook on other distribution modules currently under development.

We have developed and characterized DBR lasers emitting at 679nm and 707 nm, specifically tailored to excite the repumping wavelengths in Sr optical lattice clocks. They achieve a FWHM linewidth of about 1MHz at optical output powers of more than 65mW.

We present the design and functionality of our two distribution modules that form part of a compact and robust optical strontium lattice clock. Our modules rely on our inhouse developed ultra-compact optical isolators.

We present the layout of a miniaturized laser distribution module as part of a 88 Sr optical lattice clock. It consists of distributed Bragg reflector lasers (679 and 707 nm) combined with specifically developed, miniaturized electro-optical components.

We present an experimental setup to investigate the sensitivity of ridge waveguide semiconductor optical amplifiers to optical feedback. We determine the amplification in forward and backward directions at 767nm at varying seed and feedback powers.

Optical atomic clocks based on the spectroscopy of strontium (Sr) atoms currently provide the most accurate time and frequency measurements. They reach fractional frequency uncertainties in the range of 10 −18 [1], which makes them promising candidates for the redefinition of the second and for improving, for example, the accuracy of satellite navigation.

We present a monolithically integrated extended cavity diode laser at 778 nm with a 3 dB linewidth of 200 kHz @ 1 ms. This is the first successful demonstration of active layer removal in AlGaAs by a 2-step epitaxy manufacturing process.

The phase noise caused by current fluctuations in single-pass semiconductor optical amplifiers is experimentally investigated. Using low-index quantum barrier designs in prototype devices is found to reduce phase noise by 18% compared to reference designs.

The authors report on design, fabrication, and electro-optical characterization of single-frequency diode lasers emitting around 696, 707, and 712 nm. This has been achieved by a variation of the periods of the 10th order surface Bragg gratings implemented on a single epitaxial wafer. Depending on the wavelength, the devices achieved an optical output power of up to 60 mW at a current of 150 mA and spectral linewidths of about 400 kHz (full width at half maximum (FWHM)) and 20 kHz (Lorentzian).

The spectral linewidth of semiconductor lasers is a crucial performance parameter in a growing number of applications. A common method to improve the coherence of the laser relies on increasing the optical cavity length by an extended section without gain material. Here, this extended cavity diode laser (ECDL) concept is realized in a monolithic device at 1064 nm wavelength. This is accomplished by applying a two‐step epitaxy in the aluminum gallium arsenide material system to selectively remove the active layers in a specific section of the chip. The extended passive section decreases the 3 dB linewidth to 32 kHz at 1 ms integration time. A direct comparison between the performance of the monolithic ECDL and a distributed Bragg reflector laser from the same wafer demonstrates the feasibility of the approach. The results in terms of frequency noise, side mode suppression ratio, injection current threshold, and slope efficiency show that the passive section reduces the laser linewidth while the additional interfaces within the laser cavity created by the manufacturing process do not deteriorate the electro‐optical properties or frequency stability. This opens the possibility for the realization of gallium arsenide based photonic integrated circuits by monolithically combining active and low‐loss passive waveguides.

We present a novel approach for GaAs-based DBR diode lasers with an extended cavity. The developed chips exhibit a record small 3 dB linewidth of 25 kHz @ 1 ms at the wavelength of 1064 nm.

We report on a transportable and easy-to-operate optical clock utilizing the 2S1/2 - 2D3/2 transition of a single trapped 171Yb+ ion at 436 nm. Developed within a pilot project for quantum technology in Germany lead by industry, the clock is set up in two 19″ racks. In this way, transportation can easily be realized, and the large degree of automatization allows for operation outside highly specialized laboratories for applications beyond basic research. Comparisons to existing high-accuracy optical clock systems enable a verification of the clock's stability and uncertainty budget at the low 10−17 level. During these tests, operation with 99.8% availability over more than 14 days has been achieved.

A micro-integrated laser module has been developed for the deployment in a compact, transportable 171Yb+ optical clock. With this laser module, the clock laser system demonstrated a Modified Allan Deviation of less than 1.5×10-15 for 1 s ≤ 100 s when compared against a superior reference laser. © 2020 The Author(s)

This paper presents a GaAs-diode laser-based hybrid integration approach for the realization of very complex, though compact and robust laser modules for QIST applications in the field and in space.

We report results of numerical simulations of the dynamic properties of single-transverse mode diode lasers subject to an external optical feedback provided by a volume holographic Bragg grating. We use the traveling-wave model for the simulation and analysis of the nonlinear dynamics of the considered devices. We show that the numerical results obtained for a sample device are in a good agreement with the experiment. We investigate the influence of relevant device parameters, such as front and rear facet reflectivities and intra-cavity transmission losses on the laser behaviour.

The Master-Oscillator-Power-Amplifier (MOPA) is a laser light source best suited to provide high power, stable frequency, and narrow linewidth emission. In state-of-the-art MOPA systems, semiconductor optical amplifiers (SOAs) with single-mode lateral waveguides provide a compromise between the demands for high power on one side and excellent beam quality and small astigmatism of the optical mode on the other. The amplified spontaneous emission (ASE) in SOAs remains a limiting factor for the deployment of the MOPA systems in quantum technology applications. The presence of ASE reduces the carrier density and hence the device efficiency, increases the noise in the output signal, and adds incoherent background radiation that is critical, for example, for atom interferometry applications. It is therefore important to understand the dependence of the ASE on the design and operating conditions of an SOA in detail in order to develop SOAs optimized for applications that require spectrally very pure radiation.

Semiconductor laser technology has shown to be a promising alternative to other laser technologies, e.g. solid state lasers, for employment in coherent satellite communication and fundamental physics applications. Being compact, robust, energy- and cost-efficient these semiconductor lasers fulfill the key mandatory requirements for deployment in space. At the Ferdinand-Braun-Institut, we have developed a versatile technology platform for integration of any two semiconductor chips, be they active (e.g. laser, amplifier) or passive (e.g. phase modulator), in a fiber coupled laser module wherein all components are either space qualified or space compatible. Here we present a narrow linewidth high power laser module based on this platform that will be used in an iodine-based optical reference onboard a sounding rocket. We discuss its electro-optical performance. We further give an outlook on an ultra narrow linewidth technology demonstrator laser module.

A flexible method to measure the modulation efficiency and residual amplitude modulation, including non-linearities, of phase modulators is presented. The method is based on demodulation of the modulated optical field in the optical domain by means of a heterodyne interferometer and subsequent analysis of the I&Q quadrature components of the corresponding RF beat note signal. As an example, we determine the phase modulation efficiency and residual amplitude modulation for both the TE and TM modes of a GaAs chip-based phase modulator at the wavelength of 1064 nm. From the results of these measurements, we estimate the linear and quadratic electro-optic coefficients for a P-p-n-N GaAs/AlGaAs double heterostructure.

Recently, there has been a growing effort towards building very compact photonic devices for atomic spectroscopy in the field and in space [1]. Phase modulators are central building blocks of these devices. They are required for frequency modulation spectroscopy applications as well as for ultra-fast phase control in optical phase-locked loops. Integration of phase modulators into hybrid laser and spectroscopy modules allows for very compact and robust systems. The unique integration properties of III-V compound semiconductors with existing electronics make the realization of AlGaAs/GaAs double heterostructure phase modulators very attractive [2].

An integrated Mach-Zehnder intensity modulator for laser radiation at the wavelength 780 nm is demonstrated for the first time. The device features a double heterostructure GaAs/AlGaAs electro-optic phase modulator. The estimated insertion loss is less than 2.5 dB and the extinction ratio is 3.3 dB.