ECDL
A typical laser is constructed by forming a reflective cavity around some type of active medium. In the laser diode the active medium is a semiconductor p-n junction. When a forward bias voltage is applied to the semiconductor electrons and holes will recombine at the junction and release recombination radiation. A population inversion can be created between the energy levels of the electrons and holes allowing for stimulated emission of this recombination radiation. If the gain exceeds the losses in the junction then the laser oscillation at the resonant frequencies of the cavity will ensue. By placing the diode in an external cavity we can tune the laser frequency by changing which frequencies are resonant. The laser will ultimately run at the range of frequencies for which the gain is highest.
The Littrow angle ECDL design consists of a laser diode, a reflective diffraction grating, a mirror, and piezoelectric element; these components are arranged as shown in Fig. 1. In this design the first order diffracted beam is fed back to the laser diode forming the external cavity. The wavelength of the first-order diffracted beam is given by In a diode laser consists of a PN-junction in a semiconductor. A ridge is etched on top of the pn-junction to produce a grated index waveguide which restricts the light laterally to a single spatial mode. The front and back faces are polished. Due to the huge refractive index of the semiconductor material the faces act as mirrors and form a Fabry-P ́erot cavity. Usually the facets then are coated for passivation and to enhance the outcoupling efficiency by reducing the reflectivity to 5 % – 15 %. The gain bandwidth of such laser diodes usually spans ≈ 5 − 10 nm. If operated over threshold they usually emit laser light in one or a few longitudinal modes close to the gain spectrum’s maximum. The free running linewidth is then on the order of 10 MHz for short (ms) measuring times. To narrow the linewidth and to ensure single mode operation we constructed an external cavity diode laser (ECDL) [22] and applied additional optical feedback to the diode via an holographic reflection grating. The first diffraction order reflected off the grating is mirrored back into the output facet of the laser diode making the laser resonator quality highly frequency
ECDL design
Gimby's version
Components
| Class | component | Part number | Specification | Shop | Price | Amount | In stock |
|---|---|---|---|---|---|---|---|
| Optics | Halographic diffraction grating | 43775 (#43-775) | 1800 Grooves/mm, 12.7mm Square, VIS Holographic Grating | Edmund optics | $87.50 | 1 | |
| Electronics | Piezo-actuator | ||||||
| Electronics | OEM Laser Diode and Temperature Controller | ITC102 | 820$ |
Steck's laser
Components
| Class | component | Part number | Link | Price | Amount | Ordered | Received/In stock already |
|---|---|---|---|---|---|---|---|
| Electronics | Piezo-actuator | NAC2003-H10 | Noliac | 288 USD | 1 | Yes | |
| Electronics | Thermoelectric Cooler, Peltier | Laird 56460-501 | digikey | 37.41 CAD | 2 | Yes | Yes |
| Electronics | D-Sub 15 | SPC15307 | digikey (preferably) newark | 3.57 USD | 1 | Yes | Yes |
| Optics | Collimation Tube | LT230P-B | thorlabs; | 124 USD | 1 | x | Yes |
| Optics | Aspheric lens | 1 | |||||
| Misc | Thermal Paste | AS5-3.5G | amazon | 9.79 CAD | 2 | x | Yes |
| Electronics | Thermistor | B57861S0503F040 | digikey | 2.78 USD | 1 | ||
| Optics | Grating | 33025FL01-320H | newport dynasil | 59 USD | 1 | ||
| Optics | Anamorphic prism pair | PS871-B - N-SF11 | thorlabs | 152 USD | 1 | x | Yes |
| Electronics | Temperature Sensor | AD590 | digikey | 1 | x | Yes | |
| Optics | Sapphire Window | W7.87 | thomasnet | ? | 2 | Yes | |
| Mechanics | 4-40 x[different lengths] stainless stell socket-head cap screw | ASK440 | amazon; ebay | 12.69 USD | 1 | Yes | Yes |
| Mechanics | 8-32 x 0.375" stainless steel socket-head cap screw | JB0124FCP | amazon; ebay | 5.49 USD | 1 | Yes | Yes |
| Mechanics | 4-40 x 1 Socket Head Cap Screws Nylon | NSC0416-9A2 | fastener-express | 3 | 6 | Yes | Yes |
| Mechanics | 3/8"-24 UNF 7/8 length | Yes | Yes | ||||
| Mechanics | Adjustment Screw | 9376-K | newport | 34.10 USD | 1 | ||
| Misc | High Temperature Epoxy | EPO-TEK 353ND / Celvaseal® Leak Sealant | fiberoptics4sale / thorlabs / huntvac | 88.50 USD / 71.25 USD | 1 | ||
| Misc | Vacuum Epoxy | Hysol 1C | amazon / duniway / huntvac | 18.00 USD | 1 | ||
| Mechanics | O-Ring | 1201T23 | mcmaster | 6.97 USD | 1 | ||
| Mechanics | O-Ring | 9464K104 | mcmaster | 4.22 USD | 1 | ||
| Mechanics | O-Ring | 9464K115 | mcmaster | 3.68 USD | 1 | ||
| Mechanics | O-Ring | 1201T827 | mcmaster | 5.46 USD | 1 | ||
| Optics | Microscope glass | x | Yes | ||||
| Electronics | Kapton coated wires | accuglassproducts | 40.00 USD | ||||
| Vacuum/Mechanics | Seal off valve | ||||||
| Vacuum/Mechanics | Operator | ||||||
| Vacuum/Mechanics | Epoxy | ||||||
| Rubber isolator |
To be done
- order kapton wires
- Order Vacuum/high Temperature epoxy
Assembly procedure
- clean laser components
- Install Peltier elements
Yellow ECDL
We use it for the cavity lock. At this moment (22d of August) we have eagle yard diode, which generates TM01 mode with two almost equal polarizations. We feel that our design of the laser makes random the alignment, since any small rotation of the diode changes feedback dramatically.
The diode installed is eagle yard EYP-RWE-0810-03010-1300-SOT02-0000 with S/N: AD-18808 EYP-RWE-0810
["[file:///home/eugene/check-1.jpg file:///home/eugene/check-1.jpg]" Kot]
Alignment
Making large mode-hope free tunning range
No matter what the requirement is, it is always a good practice to first check the wavelength of the bare laser diode (without external cavity) and tune the temperature so that the wavelength is within a couple of nm of the required wavelength. From experience, staying between 15C to 30C works best, so better choose the laser diode accordingly. One should then check how the wavelength of the bare diode changes with tuning the current (\beta). After these, the diode can be put in the external cavity and the PZT ramped (without feed forward) to find \alpha. Then the feed forward should be applied in a manner explained in the paper [1]. Aim for getting around 10 GHz MDF tuning first and then slowly try to increase it by adjusting the feed forward, the current offset and the PZT voltage offset.
If you do not need a lot of optical power then always choose a laser diode whose maximum output power is also low but serves your purpose. For higher power laser diodes the length of the laser chip is longer which make the internal cavity modes closely spaced that can hinder the MDF tuning.
General idea
Laser diode = LD.[1]
We will discuss mechanical parts of our laser and how to put them together here: laser assembly. Each laser requires couple of electronic components list of which with descriptions could be found here (electronic parts of ECDL), and at last the sequence of steps for frequeny tuning is Laser feedback adjustment.
List of cites
- A compact grating-stabilized diode laser system for atomic physics. L. Ricci, M. Weidemiiller, T. Esslinger, A. Hemmerich, C. Zimmermann, V. Vuletic, W. Kijnig, T.W. Haensch 1995
- Construction of a Rb 2D-line specific compact grating-stabilized diode laser P.Gimby, A. Lvovsky, report-2007
- Design of a Mode-Hop-Free External Cavity Diode Laser for Experiments in Rubidium Spectroscopy Brian Andrade, A. Lvovsky, report-2015
- ↑ 140-GHz mode hope tuning with external cavity diode laser, arxiv
