NL2003913A

NL2003913A - Frequency stabilized laser and method for stabilizing a three mode laser.

Info

Publication number: NL2003913A
Application number: NL2003913A
Authority: NL
Inventors: Jonathan David Ellis; Ki-Nam Joo; Eric Steve Buice; Josephus Wilhelmus Spronck
Original assignee: Univ Delft Tech
Priority date: 2009-10-01
Filing date: 2009-12-07
Publication date: 2011-04-04
Also published as: NL2003913C2

Description

Frequency stabilized laser and method for stabilizing a threemode laser

The invention relates to a frequency stabilized lasercomprising in a closed loop a laser operating with a centralmode and two outer modes, a polarizer, a detector, a controller,and an actuator wherein the actuator is embodied to change thelaser cavity length so as to stabilize the operational frequen¬cies of the laser source. The invention further relates to amethod for stabilizing such a laser operating with three modes.

A frequency stabilized laser is disclosed in the arti¬cle Three-longitudinal-mode He-Ne laser frequency stabilized at633 nm by thermal phase locking of the secondary beat frequency,by Jin Yong Yeom and Tai Hyun Yoon, Applied Optics, volume 44,number 2, pages 266-270.

Such a frequency stabilized laser is also known fromthe article Frequency and power stabilization of a three longi¬tudinal mode He-Ne laser using secondary beat frequency, by H.S.Suh et al, Applied Physics Letters 63(15), 11 October 1993,pages 2027-2029.

These citations disclose a laser operating with threemodes, in which there is a low-(vx), central-(vc) , and high-(vh)frequency mode at which the laser source operates. According tothe cited art it is because of the polarization anisotropy ofthe reflecting mirrors of the laser source, that different phaseshifts on reflection for different polarizations occur, whichlifts the degeneracy of inter-mode beat frequencies, resultingin two different inter-mode beat notes, i.e. vibi= vc - vi and vib2=Vh - vc. The concurrent existence of these two inter-mode beatnotes again causes interference between them, resulting in thenonlinear electronic generation of secondary beat notes vibi -Vib2. In the known frequency stabilized laser this secondary beatfrequency is detected by a detector that is located at the rearside of the He-Ne laser through a 45° linear polarizer, passingthrough and interfering all three modes. The measured frequencyis subsequently converted to a voltage which is compared with areference voltage, and the voltage-difference is used as a con¬trol signal in a loop filter to control the cavity length of thelaser source by using two thin-film heaters connected in seriesand wrapped around the laser plasma tube.

It is an object of the invention to provide a methodfor stabilizing a laser operating with three modes, i.e. a cen¬tral mode and two outer modes, and to provide a frequency stabi¬lized laser which uses a simple measurement scheme and which hasa relatively high output power available.

It is a further object of the invention to be able touse a low-speed (100's kilohertz) detector and electronics forcontrolling the laser cavity length so as to economize on equip¬ment costs and to provide a system that is more convenient forthe user.

The method for stabilizing a laser operating with threemodes and the frequency stabilized laser of the invention areembodied in accordance with one or more of the appended claims.

Essentially the invention requires that the polarizeris applied at an angle which is selected aligned to the polari¬zation state producing the central mode of the laser, or is se¬lected aligned to the polarization state producing the two outermodes of the laser.

The invention is based on the insight that close to thefrequency modes of the laser source there are additional mixedfrequencies with absolute values vc-vh, and vh-vb. By using a main frequency mode and its adjacent mixed frequency, it ispossible to obtain a signal proportional to the laser's absolutefrequency to subsequently control the cavity length andtherewith the operating frequency of the laser source reliablyand accurately. By making use of these closely adjacent frequen¬cies it is further possible to employ a low speed detector andelectronics, which is cheaper and more convenient for the user.

A first preferable embodiment of the frequency stabi¬lized laser of the invention has the feature that the polarizeris aligned to the polarization state producing the central modeof the laser so as to pass on to the detector a central fre¬quency mode of the laser source together with its accompanyingcentral mixed mode signal that depends on interference of thecentral frequency mode with the two outer frequency modes of thelaser, and to block the two outer frequency modes of the laserand their accompanying mixed mode signals. Because the centraland central-mixed modes have differing polarization states fromthe other two modes and their adjacent two mixed mode signals,the latter frequencies can be isolated by employing the polar¬izer at a pre-established angle using standard optical compo¬ nents. Then, the interference between the central mode and thecentral-mixed mode can be detected, which produces a signal inthe order of 100's of kiloHertz. This signal is correlated tothe relative frequencies of the three modes in the laser. Whenthe laser cavity length changes, these relative frequencieschange as well. Accordingly, by detecting this interference-signal, the relative frequencies of the laser source can be sta¬bilized by stabilizing the laser cavity length.

A second alternative embodiment of the frequency stabi¬lized laser of the invention has the feature that the polarizeris aligned to the polarization state producing the two outermodes of the laser so as to pass on to the detector the twoouter frequency modes of the laser source together with theiraccompanying mixed mode signals that depend on interference ofthe central frequency mode with the two outer frequency modes ofthe laser, and to block the central frequency mode of the laserand its accompanying mixed mode signal.

Both embodiments provide an effective measurement signalfor the control loop that stabilizes the operating frequency ofthe laser source. By proper tuning of the controller andactuator that controls the laser cavity length, the laserfrequency can effectively be stabilized. The controller is forinstance a Proportional-Integral-Derivative (PID) controller.

General Description of the drawing

The invention will hereinafter be further elucidated withreference to the drawing showing in: -figure 1 power of the three modes of the laser source inthe frequency domain; -figure 2 schematic of the control loop for a frequencystabilizing of the laser source; -figure 3 power of the selected frequencies with the polar¬izer at an angle of 0°; -figure 4 power of the selected frequencies with the polar¬izer at an angle of 90°; -figure 5 schematic of a laser-based system including het¬erodyne frequency generation using a thermal actuator to controlthe laser cavity length; and -figure 6 schematic of a laser-based system including het¬erodyne frequency generation using a mechanical actuator to con- trol the laser cavity length.

Detailed Description of the drawing

When a laser operates with three modes, it exhibits thefrequency profile which is schematically shown in Figure 1. Theprofile of such a laser contains three main modes, v1( vc, andVh, and three mixed modes, vi+vb, vc-vb, and vh+vb. In general,each main mode has an associated mixed mode, as shown in Figure 1. Additionally, adjacent main modes are orthogonally polarizedwith respect to each other (parallel for and Vh, perpendicularfor vc) and the modes alternate polarization during consecutivemode hops in the same direction. Each mixed mode has the samepolarization state as its associated main mode.

To stabilize the frequency of the laser 1 shown in figure 2, a signal is generally needed from the laser which varies as afunction of absolute frequency. According to the invention, theabsolute laser frequency is stabilized by either in a first em¬bodiment blocking the outer and outer-mixed modes, or in a sec¬ond embodiment by blocking the central and central-mixed modes.

When the outer and outer-mixed modes are blocked, the cen¬tral mode (vc) and central-mixed mode (vc-vb) that are shown infigure 3 can be detected as shown in Figure 2. Typically, a po¬larizer 2 or an optical isolator can be used to block the unusedmodes. The signal that is detected with detector 3 can then beused in a controller 4 and feedback loop 5 to stabilize the la¬ser 1 cavity length with actuator 6 which stabilizes the abso¬lute frequency.

As already mentioned when the central mode (vc) and cen¬tral-mixed mode (vc-vb) are used for detection and the othermodes blocked, the three mode profile generally looks like theschematic shown in Figure 3. Since the frequencies (vc) and (vc-vb) are too high to detect, the signal used for feedback is thedifference, vb. By stabilizing this signal, the absolute fre¬quency of the laser can be stabilized.

Alternatively when the central mode and central-mixed modeare blocked, the other modes can be used for detection and thethree mode profile generally looks like the schematic shown inFigure 4. Since the frequencies, (vi) , (Vh) , (vi+vb) , and (vh+vb)are too high to detect, the signals that are detected are at frequencies (vb) and (ν^-νχ) , which have several orders of magni¬tude difference in frequency. By proper filtering, the stabiliz¬ing signal vb can be obtained and the absolute frequency of thelaser can be stabilized.

Detailed Description of preferred applications

Based on the interferometry system described in Interna¬tional application PCT/NL2009/050541, incorporated herein byreference, it is preferred to have a stabilized laser and ameans to generate heterodyne signals from the stabilized laserwith known differing frequencies. This is known as heterodynefrequency splitting in the source. In laser interferometry,typically two beams are used, which are coaxial, orthogonallypolarized, with slightly differing frequencies. Due to manufac¬turing and alignment tolerances, it is beneficial to have a sys¬tem which is free from source mixing, which adds to optical mix¬ing, creating errors.

The interferometer according to PCT/NL2009/050541 requiresa special source which does not have any source mixing. As partof that source without mixing, it is required to stabilize thefrequency of the laser 1 which will be discussed hereinafterwith reference to figure 5 and figure 6 showing two possible em¬bodiments .

With reference to figure 5 the laser frequency is prefera¬bly stabilized by blocking the outer frequency modes in the la¬ser 1 using an optical isolator or polarizer 2. If an isolatoris used, this limits optical feedback from entering the lasercavity, specifically when fiber couplers are used. The polariza¬tion state is then rotated using a half wave plate 11 to theideal rotation angle. A portion of the beam is then split fromthe main signal using a beam splitter 6 and the signal is de¬tected using a photodiode 3. The AC portion of the signal isused in the controller 4 to stabilize the laser 1 cavity length.In this embodiment the actuator 7 that provides the controllingaction that varies the cavity length is a thermal actuator.

The DC portion of the detected signal from detector 3,while not critical to the frequency locking controller, is pref¬erably used in controller 4 (or in a separate controller) to de¬termine whether the proper polarization state is coming from thelaser 1. Reading left to right from Figure 1, the polarization states are parallel for the left two frequencies, perpendicularfor the central two frequencies, and parallel again for theright two frequencies. However, they could also be the opposite:perpendicular for the left two frequencies, parallel for thecentral two frequencies, and perpendicular for the right twofrequencies .

Since the optical isolator 2 (or polarizer) is only set upto block a specific polarization state, a controller is neededto determine whether the polarization state allowed in the sys¬tem relates to the central frequency modes or the outer fre¬quency modes. The relative optical power difference between thecentral mode and outer modes of the laser can be used as a feed¬back signal to select the proper polarization state for the cen¬tral frequency mode.

Alternatively, if the detector 3 can detect high frequen¬cies (600 MHz to 1.5 GHz), this signal can be used to determinewhether the central frequency mode is passed or the outer fre¬quency modes. If a signal is detected in the highest frequencyrange, then it signals that the outer frequency modes are passedthrough the optical isolator 2. If this high-frequency signal isnot detected, then the optical isolator 2 lets the central fre¬quency modes pass.

Once the central frequency modes have been properly passedthrough the isolator 2 and the laser 1 is stabilized, two spa¬tially separated beams can be delivered to the interferometricsystem. Accordingly the beam is split equally using a beamsplitter 8 and the two resulting beams are passed through twodifferent acousto-optic modulators 9' and 9''. These modulatorsare driven at two frequencies, fi, and f2. A diffracted beam fromeach modulator (usually the first order) is fiber coupled usingtwo fiber couplers, FCi and FC2. These are then typically sent tothe interferometer, preferably the interferometer depicted byPCT/NL2009/050541.

Instead of using a thermal actuator such as a thermo¬electric device, a piezo-electric device 10 (or other thermal ormechanical actuator) can be used to change the laser cavitylength based on the controller signal. This is advantageoussince a piezoelectric device typically has a faster responsethan a thermal actuator and does not add unwanted thermal fluc¬tuations into the system. This embodiment is shown in Figure 6which in all other aspects equates with the above description with reference to Figure 5.

By combining the frequency stabilized laser of the inven¬tion with the interferometer according to PCT/NL2009/050541, acomplete system is provided having high frequency stability(which allows for measuring longer distances with higher accu¬racy) and enough power to use it in a multi-axis system (such asa lithography system) with no detectable periodic errors fromfrequency mixing and source mixing (which would contribute er¬rors in the order of nanometers).

Claims

A frequency-stabilized laser comprising in a closed loop a laser operating with a central mode and two external modes, a polarizer, a detector, a controller, and an actuator in which the actuator is adapted to change the laser tube length for stabilizing the operational frequencies of the laser source, characterized in that the polarizer is applied at an angle selected or aligned with the polarization state of the central mode of the laser aligned with the polarization state of the two outer modes of the laser.

A frequency stabilized laser as claimed in claim 1, characterized in that the polarizer is applied at an angle which is selected aligned with the decentralized mode polarization state of the laser in order to pass the laser's center frequency mode to the detector together with a central mixed mode signal which depends on interference from the central frequency mode with the two outer frequency modes of the laser, and to block the two outer frequency modes of the laser and their accompanying mixed mode signals.

3. Frequency stabilized laser according to claim 1, characterized in that the polarizer is applied at an angle which is selected aligned with the polarization state of the both outer modes of the laser in order to latent to the detector both the outer frequency modes of the laser together with their accompanying mixed mode signals dependent on interference from the central frequency mode with the two external frequency modes of the laser, and to block the central frequency mode of the laser and its accompanying mixed mode signal.

A method for stabilizing a laser operating with three modes in a closed loop control system comprising a laser operating with a central mode and two external modes, a polarizer, a detector, a controller, and an actuator in which the actuator is arranged for changing the laser tube length to stabilize the operating frequencies of the laser source, characterized in that the polarizer is applied at an angle which is selected aligned with the polarization state of the laser's central mode, or which is selected aligned with the polarization state of the both outer modes of the laser.

A method for stabilizing a laser operating with three modes according to claim 4, characterized in that the polarizer is aligned with the polarization state of the laser's central mode to the detector's central frequency mode. by allowing a laser together with a centrally mixed mode signal which depends on interference of the central frequency mode with the two outer frequency modes of the laser, and to block the two outer frequency modes of the laser and their accompanying mixed mode signals.

A method for stabilizing a laser operating with three modes according to claim 4, characterized in that the polarizer is aligned with the polarization state of the two outer modes of the laser in order to the detector both the outer frequency modes of the laser along with their accompanying mixed mode signals which depend on interference of the central frequency mode with the two external frequency modes of the laser, and to block the central frequency mode of the laser and its accompanying mixed mode signal.