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Patent Issued for Phase Locked Sideband Beams by Bias Modulation of an External Cavity Laser Diode Having a Grating

June 18, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventors Givon, Menachem (D.N. Hanegev, IL); Waxman, Amir (Jaffa, IL), filed on September 24, 2009, was published online on June 3, 2014.

The patent's assignee for patent number 8743918 is Ben Gurion University of the Negev, Research and Development Authority (Beer Sheva, IL).

News editors obtained the following quote from the background information supplied by the inventors: "High frequency modulation of laser beams is an important tool in many fields, such as communication, atomic physics and many others. In particular, the experimental realization of a .LAMBDA.-system sets the ground for many applications of precision measurements.

"Some of these applications include atomic clocks [1, 2], magnetic sensors [3] and gravity gradiometers [4]. This .LAMBDA.-system consists of two coherent laser fields, two hyperfine levels of an atomic ground state and an excited state. Depending on the application, the coherent coupling of the two sub levels of the ground state is done by laser beams which are resonant (as in CPT) or non resonant (as in Stimulated Raman transitions) with the transition to the excited state. In both cases the beams must be phase locked and with a tunable frequency difference in the range of several GHz (corresponding to the ground state's hyperfine split energy in alkali atoms).

"Three main methods have been developed over the years for the generation of the two phase-locked beams: Direct light modulation either by AOM [5] or EOM [6, 7], optical phase locking of two lasers [8, 9] and direct current modulation of a laser diode [10]. The modulation of the DC current injected to the laser diode by an AC signal of the frequency f.sub.m produces optical side bands. The first order sidebands can then be injected to slave lasers for amplification and spectral purification. The final result is two phase locked laser beams with a frequency difference of 2f.sub.m. However, the modulation response of edge emitting diode laser decreases sharply as the modulation frequency increases.

"An alternative is to modulate the current of a vertical cavity surface-emitting laser (VCSEL) [11] which is much more susceptible to high frequency modulation but has very little power (total of 2 mw).

"In the present patent application experimental results are presented, demonstrating the modulation response enhancement of an AR coated edge emitting laser diode in an external cavity.

"By eliminating the internal cavity of the laser diode and matching the modulation frequency to the FSR of the external cavity per the present invention the modulation index is enhanced to the point of complete carrier suppression even at high modulation frequency. The result is a tunable modulation source in the range of 3 to 6 GHz, with more then 60% of the total power of the output beam concentrated in the two first optical sidebands.

"Modulation enhancement by an external resonance cavity with a mode spacing corresponding to the desired modulation frequency was already reported both for edge emitting diodes [12] and VCSEL [13]. However, as the internal resonance cavity of these diodes was not eliminated, complete carrier suppression was not demonstrated in these experiments, or complete carrier suppression was demonstrated, only when the power of the beam was small.

"The modulation enhancement of a 'regular' (FP) diode (like the one used in [12]) is compared to that of the AR coated diode, to illustrate the effect of the elimination of the internal resonance cavity. The affect of the DC current on the modulation response is also examined.

"FIG. 1 of the prior art is a schematic illustration of a resonance cavity 30 of distance L, having back reflector 1 which is a high reflectivity mirror, and an output coupler 2 which is a mirror with partial transmission.

"As used herein the specification and in the claims section that follows, the term 'FSR', (free spectral range), and the like refer to the basic resonance frequency of a resonance cavity.

"FSR is given by:

".times..times..times..times..times..times..times..times. ##EQU00001## were c is the speed of light and L.sub.eff is the effective length of the resonance cavity. L.sub.eff=L, if the space inside the resonance cavity is empty. L.sub.eff=n.times.L, if the space inside the resonance cavity is filled with a material with refractive index n. L.sub.eff=n.sub.1.times.L.sub.1+n.sub.2.times.L.sub.2+ . . . if there are several materials inside the resonance cavity.

"FIG. 2 of the prior art is a schematic illustration of a resonance cavity 30 with several materials, having several indexes n along its length L. The first material is spread along L.sub.1 and has indexes n=n.sub.1, the second material is spread along L.sub.2 and has indexes n=n.sub.2, etc.

"Lasing Frequency

"The laser will emit light at a frequency .nu. that is an integer multiplication of the FSR (resonance condition): .nu.=k.times.FSR,k=1,2,3, . . . (2)

"Typically, the value of k for an operating laser is between 10.sup.5 and 10.sup.7.

"Direct Laser Modulation

"In some lasers it is possible to introduce periodic perturbation to the amplification means (gain medium) of the laser. As a result, the laser light's spectrum will include additional components, known as sidebands. In this case the laser field is given by: E=E.sub.0e.sup.i .omega.t+E.sub.k.+-.e.sup.i(.omega..+-.k.omega..sup.m.sup.)t,k=1,2,3 . . . (3)

"Where .omega.=2.pi..nu. is the main laser angular frequency (also known as the carrier frequency), and .omega..sub.m=2.pi.f.sub.m is the modulation angular frequency. E.sub.0 is the amplitude of the carrier and E.sub.k.+-. are the amplitudes of the sidebands.

"Modulation Index (Depth)

"The amplitudes of the carrier and the sidebands are approximately propositional to the Bessel functions of the first kind: E.sub.0.varies.J.sub.0(m),E.sub.k.+-..varies..+-.J.sub.k(m) (4)

"The dimensionless parameter m is an indicator to the modulation strength. For m

"Typically, direct modulation of a laser beam is achieved by modulating the DC current supply of a laser diode with an AC current source. Changing the AC frequency directly controls the modulation angular frequency .omega..sub.m. The modulation index m is dependent on several factors: The ratio between the DC and AC power, the dissipation of AC along the diode feeding circuit, reflection of part of the AC power due to imperfect impedance matching and the susceptibility of the diode's material to the AC modulation. On top of all these parameters, as the modulation frequency goes up to the GHz range, the optical sidebands move noticeably away from the resonance condition (2), and are thus strongly suppressed by the laser resonance cavity itself, as shown in FIG. 3.

"FIG. 3 of the prior art is a graphical illustration of the suppression of the amplitude of the modulated sidebands due to the effect of the resonance condition

"There is a need for a method and a device for enhancing the modulation efficiency of lasers."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "The present invention of an FSR-matched modulated laser is a device and a method that enables modulation of high power laser beams at high frequency and with high modulation index. This method enhances the modulation response of the laser, as measured either by the modulation index or by the percentage of the power that is transferred from the un-modulated laser beam to the modulated sidebands. The modulation enhancement is optionally achieved by matching the FSR (1) of the laser's resonance cavity to the modulation frequency f.sub.m, or optionally by matching the FSR (1) of the laser's resonance cavity to f.sub.m/n, n=2, 3, 4 . . . . In both cases, all the sidebands fulfill the resonance condition (2) and are thus amplified by the laser's resonance cavity and gain material. Optionally, modulation enhancement can be achieved by matching the FSR (1) of the laser's resonance cavity or to 2.times.f.sub.m. In this case, only the un-modulated beam and the even sidebands fulfill the resonance condition (2) and are thus amplified by the laser's resonance cavity and gain material. In this case the odd sidebands do not fulfill the resonance condition (2) and are thus strongly suppressed by the laser's resonance cavity.

"Modulated laser beams find diversified usage: in optical communication, atomic clocks, magnetometers, radar systems, manipulation of quantum states in modern quantum mechanics research and many other applications. Providing a device and a method for enhanced modulation of laser beams might have a significant impact, either by providing cheaper modulated lasers or by providing additional applications of modulated laser beams

"Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

"According to the present invention there is provided an FSR-matched modulated laser with enhanced modulation efficiency, the FSR-matched modulated laser including: (a) a M-laser, the M-laser including: (i) a back reflector; (ii) an output coupler, having a mirror with partial transmission quality, wherein the output coupler is positioned facing the back reflector with a distance L between the output coupler and the back reflector, wherein the distance L is a cavity length; (iii) a gain medium disposed between the output coupler and the back reflector; and (iv) a light modulation means disposed between the output coupler and the back reflector, wherein the M-laser has a free spectral range, wherein the modulation means has a modulation means frequency f.sub.m, and wherein the modulation means frequency f.sub.m is equal to an integer times the free spectral range.

"According to further features in described embodiments of the invention described, the modulation means f.sub.m is equal to one half of the free spectral range.

"According to further features in described embodiments of the invention described, the FSR-matched modulated laser further including: (b) a modulation source including: (i) a laser power supply; (ii) a bias T; (iii) a power line, the power line feeding power from the laser power supply to the bias T; (iv) a modulation generator; (v) a bias T modulation line, the bias T modulation line feeding modulation signal from the modulation generator to the bias T; (vi) a laser feed line, the laser feed line feeding the power from the bias T to the gain medium; and (vii) a direct modulation line, the direct modulation line feeding the modulation signal from the modulation generator to the light modulation means.

"According to further features in described embodiments of the invention described, the cavity length is adjustable.

"According to further features in described embodiments of the invention described, the cavity length is adjustable, by moving of the output coupler.

"According to further features in described embodiments of the invention described, the cavity length is adjustable, by moving the back reflector.

"According to further features in described embodiments of the invention described, the refractive index of the gain medium is adjustable.

"According to another embodiment of the invention an FSR-matched modulated AR laser with enhance modulation efficiency, the FSR-matched modulated AR laser including: (a) an external cavity diode laser including: (i) an AR diode, having a front facet, a back facet, a diode gain material, and a lens, wherein the front facet is coated with anti reflection films; (ii)

"a grating, positioned such that part of an internal AR laser beam that hits the grating is reflected back and another part of the internal AR laser beam is reflected to an output direction as an output AR laser beam; (iii) a piezoelectric transducer, wherein the piezoelectric transducer is firmly connected to the grating; and (iv) a mechanical translation stage, wherein the piezoelectric transducer is firmly connected to the translation stage, wherein the piezoelectric transducer can move in a motion direction which is aligned parallel to a direction of the internal AR laser beam, wherein the FSR-matched modulated AR laser has a free spectral range, wherein the FSR-matched modulated AR laser is modulated by a modulation means having a modulation means frequency f.sub.m, and wherein the modulation means frequency f.sub.m is equal to an integer times the free spectral range.

"According to further features in described embodiments of the invention described, the modulation means f.sub.m is equal to one half of the free spectral range.

"According to further features in described embodiments of the invention described, by applying electric voltage to the piezoelectric transducer a position of the grating relative to the translation stage can be fine tuned.

"According to further features in described embodiments of the invention described, the FSR-matched modulated AR laser further including: (b) an AR modulation source including: (i) an AR modulation source, for supplying power to the gain medium; (ii) an AR modulation generator, that produces a modulation signal, which is a periodic electrical signal at frequency f.sub.m; (iii) an AR Bias T, for superimposing the power with the modulation signal; (iv) an AR power line, the AR power line feeding the power from the AR laser power supply to the AR Bias; (v) an AR bias T modulation line, the AR bias T modulation line feeding the modulation signal from the AR modulation generator to the bias T; and (vi) an AR laser feed line, the AR laser feed line feeding the power superimposed with the modulation signal from the bias T to the gain medium, wherein the supplying of the power to the gain medium is done through the AR bias T."

For additional information on this patent, see: Givon, Menachem; Waxman, Amir. Phase Locked Sideband Beams by Bias Modulation of an External Cavity Laser Diode Having a Grating. U.S. Patent Number 8743918, filed September 24, 2009, and published online on June 3, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8743918.PN.&OS=PN/8743918RS=PN/8743918

Keywords for this news article include: Electronics, Laser Diodes, Ben Gurion University of the Negev Research and Development Authority.

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Source: Electronics Newsweekly


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