Patent ID: 12255438

DETAILED DESCRIPTION

FIG.1shows the design and the function of a wavelength locker as described in the text, particularly in example 1. The top part ofFIG.1shows the outline of a fiber coupled wavelength locker approximately 40 mm×15 mm×8.5 mm. The bottom part ofFIG.1illustrates the functionality of the wavelength locker: light from the optical fiber is divided by a beam splitter (BS) into one part that acts as a reference signal (detected by photo detector PD1) and another part that is passed through a frequency discriminating element, here specifically a solid silica etalon (detected by photo detector PD2). The two photo detectors are serially connected such that the anode of PD2is coupled to the cathode of PD1—creating a balanced detection scheme.

FIG.2also shows the functionality of a generic wavelength locker illustrating the frequency selective nature of the frequency discriminating element.

FIG.3shows the transmission function of interferometers with different values of the finesse as well as an indication of a typical position of the locking point. The locking point, however, does not need to be at the 50% mark. The locking point can be adjusted by measuring the amplified signal from the balanced detection against an offset voltage value.

FIG.4illustrates how the devices and techniques of the disclosure can be used to improve the frequency noise of a tunable fiber DFB laser.

FIG.5shows an external cavity diode laser from prior art500, as e.g. shown in FIG. 2 of US2004/0101016 A1. The laser cavity comprises a high reflectivity mirror501, a tunable etalon (502), a lens (503), and a diode laser503. The back side of the diode laser act as an output coupler and the laser output is thereby going through laser diode (504).

FIG.6illustrates the use of the tunable etalon inside the external cavity diode laser shown inFIG.5. The laser gain band601overlaps with several cavity modes602and in the absence of an etalon the laser can lase on several cavity modes simultaneously. This is also referred to as that lasing at several longitudinal modes. However, the transmission spectrum of the etalon (603) has a narrow passband and thus acts as a mode selector so that the laser only lases at one laser cavity mode. The tunability of the etalon allows for fine-tuning of the laser frequency through adjustment of the laser cavity optical path length

FIG.7shows a laser device according to the disclosure. It comprises a tunable laser with a single output mode700. The output of the laser traverses an optical fiber701and an optical coupler or splitter702. One of the output ports of the coupler is laser output703. The other output is sent to a high finesse tunable etalon704and the transmission of the etalon705is sent back into the laser cavity to generate an error signal to stabilize the laser frequency.

FIG.8illustrates the etalon transmission spectrum (800) and a suitable locking point801for the tunable laser700shown inFIG.7. By designing the etalon with a high finesse and with a compact design, the etalon center frequency can be made more stable than that of the laser, so that frequency locking the laser to the etalon reduces the frequency noise of the laser.

FIG.9shows a laser device according to the disclosure where the tunable laser700consists of an external cavity diode laser500as shown inFIG.5.

EXAMPLE

An example of the embodiment of a device according to the disclosure was produced in the following way. A low noise distributed feedback fiber laser operating at 1542.5 nm and providing an output power of approximately 50 mW and a spectral linewidth of <1 kHz was fabricated according to standard production processes. A hermetically sealed custom designed wavelength locker with dimensions of approximately 40 mm×15 mm×8.5 mm was fabricated. The frequency discriminating element consisted of a solid silica Fabry-Perot interferometer with a free spectral range of 25 GHz and a finesse of approximately 50. The temperature sensor was fixed in direct contact with the interferometer. A portion of the light from the distributed feedback fiber laser is passed via optical fiber to the custom designed wavelength locker. Inside the wavelength locker, the light from the optical fiber was collimated and divided by a beam splitter into one part that acts as a reference signal detected by a reference photo detector and another part that is passed through the solid silica interferometer and detected by a signal photo detector. The two photo detectors were serially connected such that the anode of the signal photo detector was coupled to the cathode of the reference photo detector—creating a balanced detection scheme. The outline and the schematic of the wavelength locker are illustrated inFIGS.1and2. The fiber laser and wavelength locker were packaged in the same mechanical enclosure (dimensions 23 mm×92 mm×200 mm) including thermal control of both fiber laser and wavelength locker and additional thermal shielding of the wavelength locker to reduce the impact of ambient thermal variations. The fiber laser frequency can be tuned using thermal and piezo control. By locking the fiber laser to one of the steep wavelength locker slopes (using feedback to both fiber laser temperature and piezo voltage), the frequency noise of the fiber laser could be reduced by more than two orders of magnitude over a broad frequency range—as illustrated inFIG.4. Using an external modulation input it was possible to provide fast tuning of the laser without compromising the obtained improvement in frequency noise.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.

REFERENCES

R. van Leeuwen, L. S. Watkins, C. Ghosh, R. Gandham, S. R. Leffler, B. Xu, and Q. Wang., Princeton Optronics, 1 Electronics Drive, Mercerville, NJ 08619, “Low Noise High Power Solid State Laser for 1550 nm Wavelength Band”, Proceedings of the 19th annual meeting of the IEEE Lasers and Electro-Optics Society (LEOS '06), p. 336 (2006).Michael Tröbs, Luigi d'Arcio, Gerhard Heinzel, Karsten Danzmann, “Frequency stabilization and actuator characterization of an ytterbium-doped distributed-feedback fiber laser for LISA”, J. Opt. Soc. Am. B, Vol. 26, No. 5, May 2009.Jan Hald, Lars Nielsen, Jan C. Petersen, Poul Varming, Jens E. Pedersen, “Fiber laser optical frequency standard at 1.54 μm”, Optics Express, Vol. 19, Issue 3, pp. 2052-2063 (2011).Jesse Tuominen, Tapio Niemi, and Hanne Ludvigsen, “Wavelength reference for optical telecommunications based on a temperature-tunable silicon etalon”, Rev. Sci. Instrum. 74, 3620 (2003).Caroline Gréverie, Catherine N. Man, Alain Brillet, Jean Pierre Coulon, Jens Engholm Pedersen, “Stabilisation en Frequence d'un Laser Fibre par Controle du Courant de Pompe”, JNOG 2008 Conference, Lannion, France, Ma2.3.US2004/0101016 A1 (MACDONALD et al.) 20040527, “Hermetically sealed external cavity laser system and method”.WO 03/005502 A2 (New Focus Inc., Intel Corp.) 20030116, “External cavity Laser with selective thermal control”.