External Cavity Diode Laser - part 1: optical path and mechanical design

 Introduction

External cavity diode laser (ECDL) combines benefits of laser diodes and DPSS lasers. The semiconductor chip provides easy to pump, efficient, high gain medium while the external cavity allows tailoring device performance.  Wavelength selective elements, like diffraction gratings or etalons can be added to the resonator to reduce emission spectrum, saturable absorber can be added for mode-locking, partially reflective mirror in conjunction with RF modulation can be used for frequency comb generation and many more. In this article, we will try to design simplest, ECDL that'll be used in future experiments so state tuned!

How it works

There are many great internet sources describing how ECLD works, so let's just quickly recap. Out of the box laser diode has relative wide emission bandwidth, usually around 1-2nm. This is because the gain bandwidth of diode is relative wide and the resonator works with relatively weak coupling (output mirror has reflectivity of 32% for AlInGaP, 18% for GaN). To reduce this bandwidth, some wavelength selective element has to be introduced inside the resonator. Of course, we can't insert elements into the semiconductor chip (DBG laser manufacturing is beyond my capabilities).

Unfortunately, most laser diodes available on the market don't have AR coatings on facets, hence they have an internal resonator that needs to be worked around. It is usually done by providing additional, external feedback with high enough reflectivity to compete with the internal resonator. If the pump current is low enough and conditions are selected properly then external feedback works with internal one to stabilize single mode operation. One downside of such a solution is that there's higher circulating power for given output power, due to higher effective OC reflectivity and because most currently sold laser diodes are power limited by facet destruction it directly reduces maximum available output power.

Note about coupling

It's worth noting that laser diodes have a highly divergent output beam. If the output cavity is to be composed of anything but a big concave mirror, some collimation is needed. It's usually achieved using a single, aspherical collimator near a laser diode. While it provides decent collimation it doesn't account for astigmatism of the output beam. Most of the time it's not a issue as external feedback only needs to return a minority of the light back to the diode. In the case, high external coupling is needed additional cylindrical should be used to further collimate the beam.

If you'd like to read more about theory of operation of ECDL click: here here here

 

Design 

In this build, I'll use diffraction grating to reduce the bandwidth. The grating will be operating in Littrov configuration. In this case, it's beneficial to choose a grating that has the lowest grove count while providing only single diffraction order to not lose power unnecessarily. Rearranging some equations, we find that 2nd order diffraction in grating working in Littrov mode happens at the angle arcsin(3/2*lambda*grove density). If this arcsin doesn't exist, because argument is bigger than one, then grating doesn't have higher orders. For 520 nm this critical grove density is around 1300/mm. On the other hand, we would like the grating to have the lowest possible grove density for highest dispersion and easier single mode operation. My supplier stocks only 1200 and 1800 groves/mm gratings, not 1300. And while 1200 would provide some loss in higher order diffraction it's hard to judge which one will have higher efficiency, while 1200 provides higher dispersion. For testing, I decided to order both.

Grating bandwidth

 Laser diode facet size is roughly 1 μm x 5 μm and the collimator used has an effective focal length of 3mm. This means that for the feedback power to reduce by half the returned beam has to move by 0.5 μm or 2.5 μm, depending on the axis. This implies that change of angle of returning beam by just 9.5 or 47.7 milidegrees is enough to reduce feedback by half. For 1200 groves / mm grating used in this project it gives 3dB bandwidth of just 0.27nm or 1.38nm respectively.

External cavity mode spacing

When cavity round trip distance is 100 mm, then around 2e5 wave nodes fit in the resonator. Then mode spacing is 520 nm/2e5 or around 2.6 pm, much tighter than grating bandwidth. Clearly, many modes of external resonator can fit in gratings bandwidth. Fortunately, there's also an internal cavity that provides selectivity. Because the external cavity's length is defined by aluminum bracket holding parts together, it's expected to change by around 0.0023%/K. While it doesn't seem like much, it gives almost full mode hop for 1K of temperature change.

Internal cavity mode spacing

A laser diode's structure is usually a few hundred um long. For simplicity, let's assume it's 500 um long and that index of refraction is 3.5, giving 7630 wave nodes inside the resonator for a mode spacing of 77.3 pm. Overlapping both cavities and grating bandwidth creates higher selectivity filter (see picture), additionally external cavity stabilizes small drifts due to thermal changes of internal cavity. It should be noted that because of big internal cavity mode spacing, small changes to temperature or injection current will hop external cavity modes one by one retuning the system. It's therefore critical to stabilize diode temperature and current. How precisely should temperature be stabilized depends on what semiconductor is used. For eg. red laser diodes, made from AlInGaP are very sensitive to temperature, drifting 120 pm / K while free running. To maintain single external cavity mode, temperature has to be regulated to with-in 21.7 mK. For GaN diodes the requirements aren't nearly as strict. 

source: http://dx.doi.org/10.1524/zpch.2011.0182

 Physical construction

Let's look from outside to inside. The entire system is placed on big slab of aluminum working as sturdy base and heatsink. On that there's Peltier module mounted, on which rests an inner baseplate. The inner baseplate holds the entire optical path. On one right there's New Port mount to which diffraction grating will be glued, on the other end there's another Peltier module, hosting brass laser diode + collimator mount. To protect the optics from dust and air currents, everything will be covered with 3d printed plastic cover with glass window positioned at brewster angle or AR coated window, depending what will be easier to source.

the inside of the device