Diode laser alternative for detection of gravitational waves

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Figure 2: Micro-integrated ultra-narrow linewidth diode laser based on resonant optical feedback (footprint of the AlN optical bench (light grey): 80 x 30mm2)

As part of a European Space Agency project, researchers from the Ferdinand-Braun-Institut have developed a micro-integrated ultra-narrow 1,064nm diode laser for the Laser Interferometer Space Antenna (Lisa), a satellite mission that aims to detect and characterise gravitational waves.

The diode laser provides lower noise than the Nd:YAG laser used currently in the Lisa instrument, and has been put forward as a potential replacement.
To detect gravitational waves, changes in the distance between test masses on board three satellites that are spaced 2.5 million kilometres apart need to be measured in the order of a few pm.

This challenging task will involve the use of three laser interferometers, whose key elements are metrology lasers. Despite being amplified and stabilised in amplitude as well as in frequency, these lasers require very low-noise seed lasers. In previous studies, a Nd:YAG non-planar ring oscillator (NPRO) laser like the ones used for free-space optical satellite communication was selected as baseline for the seed laser.

Figure 1: principle setup of the diode laser with resonant optical feedback consisting of an external cavity (Cav), a semiconductor gain chip with AR coating on both facets, and a VHBG

As part of the European Space Agency (ESA) project, the Berlin based Ferdinand-Braun-Institut (FBH) researchers developed a potential alternative seed laser based on a diode laser operating at 1,064nm with resonant optical feedback. The laser (figure 1) consists of an external cavity, a semiconductor gain chip with an anti-reflective coating on both facets, and a volume holographic Bragg grating (VHBG).

This setup resembles a conventional extended cavity diode laser (ECDL) but with the external cavity replacing one of the mirrors. Here, the external cavity acts as a mirror and the resonant feedback is re-injected into the gain chip, if the frequency of the ECDL part matches the resonance frequency of the external cavity. As the light travels back and forth in the external cavity before feeding back to the gain chip, this setup effectively implements a very long cavity, but at the same time avoids problems related to very long conventional ECDL setups.

The ultra-narrow linewidth diode laser based on the described concept is micro-integrated on an aluminium nitrate (AlN) optical bench (footprint 80 x 30mm2), which is part of FBH’s versatile hybrid integration technology platform for implementation of complex laser systems. This platform also provides the means to couple the emitted light into a polarisation-maintaining optical fibre.

The micro-integrated module is depicted in figure 2 (top of story). The semiconductor gain chip (on its submount) and the passive optical elements like external cavity, VHBG, lenses, wave plate, mirrors, optical isolator, thin film polariser, and fibre collimator are adhesively bonded to the optical bench using FBH’s established micro-integration process.

Figure 3: Frequency noise PSD of the micro-integrated laser module derived from a self-delayed heterodyne (SDH) measurement

In order to assess the frequency stability of the laser module, the frequency noise power spectral density (PSD) of the laser was measured in a self-delayed heterodyne setup with a delay line length of 2km. The result is in figure 3. At Fourier frequencies below 10kHz, the PSD exhibits a 1/f2 behaviour. Such a slope is also common for a Nd:YAG NPRO. At the working point, the PSD of the laser module reaches a white noise floor of approximately 8Hz2/Hz for Fourier frequencies above 100kHz. This corresponds to a Lorentzian linewidth of 13Hz.

The laser module also allows for frequency tuning, the researchers noted. As the resonance frequency of the external cavity determines the emission frequency of the entire laser, frequency tuning of the laser is achieved by tuning the resonance frequency of the external cavity. For coarse and slow frequency tuning this can be realised by means of the cavity temperature.

In order to enable fast frequency tuning, the external cavity is made of LiNbO3. This allows for electro-optical tuning on timescales exceeding the capabilities of an Nd:YAG NPRO by several orders of magnitude. A locking algorithm ensuring that the optical length of the ECDL part follows this tuning was developed and verified.

In summary, the researchers created a micro-integrated ultra-narrow linewidth diode laser module, which was developed as a potential replacement of the Nd:YAG NPRO for the Lisa mission. The module achieves a Lorentzian linewidth of 13Hz.

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The MOGLabs’ cateye diode laser (CEL) offers a new twist on external cavity diode lasers. A cateye reflector and ultra-narrow filter replace the alignment-sensitive diffraction grating of conventional Littrow and Litman-Metcalf designs.

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