Diode Laser Modules
While diode lasers have been commercially integrated into simple and compact optical systems, e.g. in DVD players or telecom devices, compact systems are yet not available for applications like precision spectroscopy. Up to now, when compact and robust diode laser systems are required for spectroscopy, distributed feedback (DFB) (or distributed Bragg reflector, DBR) lasers have to be used. However, these typically do not achieve the frequency stability required for all aspects of precision optical metrology.
DFB diode lasers developed at FBH for 780 nm and 920 nm describe the state-of-the-art in DFB laser development, specifically in terms of output power and linewidth. Single emitters emitting in single longitudinal and transverse mode deliver more than 250 mW of output with a short term linewidth (10 µs) at or slightly below 1 MHz (technical noise, mostly attributed to off-the-shelf diode laser controllers). The intrinsic linewidth (due to fundamental laser frequency noise) is well below 100 kHz. Further, power amplifiers provide an output power in excess of 10 W in single transverse mode. Again, FBH is defining the state-of-the-art here.
DFB lasers providing an output of more than 200 mW at 976 nm have very recently been used at FBH for on-the-microbench second harmonic generation (SHG) and generated 56 mW at 488 nm for Raman spectroscopy applications from a 50x25x10 mm3 module. The micro-bench technology is based on mounting the semiconductor chip together with micro-optics on a common, compact (e.g. 13x4x1 mm3 ) ceramic platform. This represents the state-of-the-art for micro-integrated systems. Further, micro-integrated external cavity diode lasers developed at FBH have also very recently proven to be suitable for Raman difference spectroscopy applications, delivering more than 250 mW of power at 671 nm from a 25x25x10 mm3 module.
|DFB-MOPA integrated within a 25x25x10 mm³ volume with microbench technology. The front end of the capacitivly cooled package shows the ceramic microbench with the DFB laser coupled to the power amplifier by means of a micro-optical gradient index (GRIN) lens, FBH.|
As already mentioned above, DFB lasers typically are not suited for all aspects of ultra-high precision spectroscopy. This is owed to their linewidth, which is at least one order of magnitude larger than that of extended cavity diode lasers or other advanced laser concepts, and which typically can not be reduced by active control to the level required for all aspects of optical metrology. The state of the art in cold atom research and its applications thus rely on extended cavity diode lasers and hence, up to now, on macroscopic optics.
The integrated optics approach can not easily be applied to the laser systems, as some of the required technology like isolators and spectroscopy cells cannot yet be realized in integrated optics. iSense aims at a step-change using the novel optical microbench technology used by the partner FBH. At FBH, significant effort is currently put into getting beyond state-of-the-art and realizing narrow linewidth MOPA-lasers and ECDLs in microbench technology. Up to now, the short-term linewidth of integrated systems (MOPAs or ECDLs) has not been determined experimentally. However, it is expected that these systems reach or outperform their macroscopic counterparts if optical feedback can be avoided. More complex systems, like ECDLs with an atomic reference cell integrated on the microbench, have not been realized so far by any group. Hence, the corresponding goal defined in this proposal clearly goes beyond-state-of-the-art. Nevertheless, because the FBH microbench technology has proven to be scalable to more complex laser systems, no show-stoppers are anticipated by the consortium at present.
iSense will develop microbench optical modules including all the non-integrable parts in the smallest possible form factor – a factor of 100-1000 volume size reduction as compared to the current state-of-the-art. The novelty lies in micro-integrating these diode laser systems for ultra-high precision spectroscopy applications with the constraint that the systems have to function as specified under “back-packing” conditions.
Research progress in the 1st year
The micro-integrated laser production is well in schedule. DFB-RW lasers have been produced with the required parameters. The power amplifier wafers have been processed and we are awaiting first measurements. The development and design of extended cavity diode lasers, which have a narrow linewidth, has been completed and components sent out to manufacture. First prototypes are expected in late August 2011. Miniaturized spectroscopy cells, which allow the lasers to stay tuned to the atomic resonances, have been produced and tested. Fibre coupling methods and Zerodur baseplate technologies have been tested on track for integration of the entire laser system.