Single Angle Facet (SAF) Gain ChipsFor External Cavity Diode Lasers, It All Begins with the Gain Medium

Photodigm Brings Its Leading Edge Process Technology to External Cavity Diode Lasers

Photodigm single angle facet (SAF) gain chips are designed for use in  external cavity diode lasers (ECDL). ECDLs are like any diode laser, consisting of a semiconductor gain medium in a cavity. Like a DBR laser, they include a grating within the cavity, outside the gain region, to select a single longitudinal mode.  Unlike a monolithic DBR laser diode, the cavity is extended beyond the chip itself to allow for an external grating and a longer cavity length.  The longer cavity length can result in significantly reduced linewidth, while the lasing wavelength can be tuned by adjusting the grating angle. 

Conventional ECDLs usually use a straight ridge Fabry-Perot laser with high reflectivity (HR) facet and a modified anti-reflective (AR) facet.  By making the AR facet of sufficiently low reflectivity, feedback is reduced to such a low point that the device does not lase but only emits amplified spontaneous emission (ASE).  Such a device with an HR coated facet and an AR coated facet will not normally lase. However, because the semiconductor has very high gain, even small amounts of reflective feedback can initiate lasing and the broad ASE spectrum (20 nm or more) collapses to a narrow lasing spectrum.  This limits the power that can be obtained from the gain chip during normal ECDL operation.

Angling one of the facets relative to the rest of the gain ridge, as seen in the Figure below,  suppresses feedback into the semiconductor by as much as 4 orders of magnitude.  As a result the gain chip emits only ASE.  Furthermore with such low feedback, the SAF gain chip can operate at higher powers and still produce an extremely low noise ASE spectrum, making it the ideal solution for high performance external cavity diode lasers. 

Single Angle Facet chip for External Cavity Diode LasersSchematic of Single Angle Facet gain chip ridge design.  

Photodigm produces the world’s highest performance single frequency DBR lasers in the 740-1100 nm spectral range.  Monolithic DBR lasers and hybrid external cavity diode lasers are complementary devices.  Both produce narrow bandwidth, tunable, single frequency laser emission.  DBRs tune over about 2 nm, limited by the bandwidth of the DBR reflectors.  This is sufficient for many applications. ECDLs tune over as much as 20 nm, dependent on the gain bandwidth of the semiconductor.  ECDLs with their longer cavity lengths can have lower linewidths than DBRs, although the <500 kHz linewidth of the DBR is sufficent for most applications.  However, monolithic DBRs require minimal alignment and setups are foar more robust in terms of vibration and noise immunity. Furthermore, DBR laser chips are highly scalable and follow semiconductor economics with dramatically falling costs as production volumes rise.  ECDLs may be valued where flexibility is more valued than robustness, or where the narrower linewidths and larger tuning range offer advantages.  A comparison of DBRs and ECDLs is shown in the accompanying table.


DBR Laser


Tunability range

2 nm

Up to 20 nm or more


<500 kHz



>200 mW

<100 mW


<0.25 cc in package

10’s of cc’s





Photodigm developed its SAF gain chip product line as an extension of its DBR laser product line.  Working within our proprietary wafer fab facility, we have developed all the key process technologies required for SAF gain chips through our extensive experience in DBR lasers. We are also dedicated to serve the needs of our customers for high performance lasers in the areas of spectroscopy and metrology.  While in the long run we believe that the mechanically robust, monolithic DBR laser architecture will replace external cavity diode lasers in many applications, particularly in mobile devices and commercial products, we also realize that some applications will continue to prefer ECDLs.  These applications include laboratory researchers, who value the unique characteristics of flexible tunability and extremely narrow linewidths that external cavity diode lasers provide.   

We currently supply SAF gain chips in the following wavelengths:  

  • 780 nm.  See data sheet PH780SAF1009MM
  • 795 nm   See data sheet  PH795SAF1009MM
  • 850 nm   See data sheet PH850SAF1009MM
  • 1080 nm See data sheet PH1080sAF1009MM

 These devices are available to both OEMs and end users for retrofit into existing external cavity diode lasers.  Please contact Photodigm for more information.