Pound Drever Hall Lock-In - Revision history

Wikiuser: /* Future Plans */

2014-06-12T21:22:30Z

‎Future Plans

Wikiuser: /* Past Progress */

2014-06-12T21:06:52Z

‎Past Progress

Wikiuser: /* Setup */

2014-06-12T20:46:38Z

‎Setup

Wikiuser: /* Setup */

2014-06-12T20:43:32Z

‎Setup

Wikiuser: /* Theory */

2014-06-12T20:43:04Z

‎Theory

Wikiuser: /* Our Setup */

2014-06-12T20:40:07Z

‎Our Setup

Wikiuser: /* Our Setup */

2014-06-12T19:42:18Z

‎Our Setup

Wikiuser: /* Our Setup */

2014-06-12T19:37:12Z

‎Our Setup

Wikiuser: /* Our Setup */

2014-06-12T19:36:41Z

‎Our Setup

Wikiuser: /* Our Setup */

2014-06-12T18:55:41Z

‎Our Setup

← Older revision		Revision as of 21:22, 12 June 2014
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	==== Future Plans ====		==== Future Plans ====
−	The next step in this project is to optimize the laser sidebands. Currently the nonuniform of the sidebands is so great that the signal from the cavity is over powered. To solve this problem both the sidebands and the cavity signal must be optimized. The optimization of the sidebands will allow the sidebands to cancel out, or cancel more than they are. By optimizing the cavity we will be able to discern the sidebands effect on the light stored in the cavity	+	The next step in this project is to optimize the laser sidebands. Currently the nonuniform of the sidebands is so great that the signal from the cavity is over powered. To solve this problem both the sidebands and the cavity signal must be optimized. The optimization of the sidebands will allow the sidebands to cancel out, or cancel more than they are. By optimizing the cavity we will be able to discern the sidebands effect on the light stored in the cavity. With these corrections the beat frequency should correspond to the laser-cavity frequency difference.

@@ Line 35: / Line 35: @@
 <br />
 With the Cavity tested, we moved to the diode laser Bias Tee. The Bias Tee that was installed in the custom diode laser apparently had an inductor and a capacitor in series. This is assumed as an error as the inductor was an adjustable one. In the end we rebuilt the Bias Tee onto the laser driver circuit. The only problem with the driver circuit was that it was made to output 250mA, which will easily overdrive our laser diode. We tried to limit the max current, but failed to find a way to modify the circuit. To solve this problem we decided to buildNo past progress as this project just started our own current source. This ended well, but while we were trying to have a negative current source, we misused the LM317 which is a "positive" voltage regulator. To solve this we require a negative adjustable voltage regulator, the LM337.
-[[File:Laser driver.png|thumbnail|Laser Driver Circuit (Supplies Current)]]
+[[File:LD_Current_Supply.jpg|thumbnail|Laser Driver Circuit (Supplies Current)]]
 ==== Current Progress ====

@@ Line 13: / Line 13: @@
 [[File:Fabry-Perot-Setup.bmp|thumbnail|Test Setup]]
-[[File:EXperimental_setup-bmp13|thumbnail|Beat Frequency Detection Setup]]
+[[File:EXperimental_setup-bmp13.jpeg|thumbnail|Beat Frequency Detection Setup]]
 Our former setup was configured as a Fabry-Perot Interferometer. This setup was used to verify that our Fabry-Perot Cavity was tuned and aligned to the laser. This past setup was to align the Cavity to a specific beam path that another laser could be aligned to. The main reason for having an alignment laser was because our diode laser had a bad mode structure and was low power, by using a Helium-Neon laser we are able to align to Cavity to a high finesse without the difficulty of having to deal with bad modes. Also note that M2 is a flip mirror and the there is another set of beam steering mirrors behind it for the diode laser, which is not shown.

@@ Line 13: / Line 13: @@
 [[File:Fabry-Perot-Setup.bmp|thumbnail|Test Setup]]
-[[File:EXperimental_setup-bmp12|thumbnail|Beat Frequency Detection Setup]]
+[[File:EXperimental_setup-bmp13|thumbnail|Beat Frequency Detection Setup]]
 Our former setup was configured as a Fabry-Perot Interferometer. This setup was used to verify that our Fabry-Perot Cavity was tuned and aligned to the laser. This past setup was to align the Cavity to a specific beam path that another laser could be aligned to. The main reason for having an alignment laser was because our diode laser had a bad mode structure and was low power, by using a Helium-Neon laser we are able to align to Cavity to a high finesse without the difficulty of having to deal with bad modes. Also note that M2 is a flip mirror and the there is another set of beam steering mirrors behind it for the diode laser, which is not shown.

@@ Line 9: / Line 9: @@
 [[File:Applications PDH scetch.png|thumbnail|Basic Cavity Locked to Laser]]
 The main concept behind the PHD lock-in method is the use of a Cavity to sense the laser frequency and then adjust the laser based on the laser-cavity frequency difference. The basic way of using a Cavity to measure a lasers frequency is by measuring the transmission or reflection of the cavity at different cavity lengths. Since, the objective is to set the laser to a stabilize a laser around a single frequency, the cavity length will not change. The frequency measurement should also be from the reflected beam of the cavity, so transmitted beam may used. If the cavity resonance is near the laser wavelength, then the reflectance can be approximated by a quadratic function. Though, you can't lock a laser to just the reflected beam because if the laser is slightly off resonance with the cavity there is no way to tell if the laser frequency is higher or lower than the cavity, you can only tell that the laser is off resonance. To fix this note that the derivative of a quadratic function is linear and therefore would fix the frequency ambiguity of the quadratic function. Now, in order to sense the derivative of the reflected beam, with respect to frequency, the laser needs to have is frequency varied. The reason for varying the laser frequency, is to allow for a discrete approximation of a derivative. This means that the reflected beam will have three frequencies, each with there own intensities, based on the reflectance at the respective frequencies. Therefore, when the reflected beam hits the detector, the detector will detect a beat frequency. This beat frequency will create AC signals based on the frequency interaction. In order to know what side of the cavity resonance the laser is on, an analysis of the cavities effect of the lasers side bands is needed. The detector will output quite a few frequencies, but the ones of interest are oscillating at frequency that the laser is being driven at, i.e. the side band beat frequency. to turn these AC signals into a DC signal the use of a mixer is needed, this is to turn the desired ac frequency into (ac)^2. This trick allows the use of the Trig Identity Cos(wt)^2 = 1 + Cos(2wt), meaning that with the addition of a low pass filter, a DC signal based of the magnitude of the the difference of the center frequency and the sideband that gets reflected the most can be obtained.
 [[File:Fabry-Perot-Setup.bmp|thumbnail|Test Setup]]

@@ Line 18: / Line 18: @@
 ) The laser is on the center frequency and the sidebands cancel out.
 ) The laser frequency is lower than the cavity center frequency and the lower sideband interacts with the stored laser frequency, making a beat frequency.
 ) The laser is frequency is greater than the center frequency of the cavity, then the upper sideband interacts with the stored laser frequency.

@@ Line 13: / Line 13: @@
 [[File:Fabry-Perot-Setup.bmp|thumbnail|Test Setup]]
-Our current setup is configured as a Fabry-Perot Interferometer. This setup was used to verify that our Fabry-Perot Cavity was tuned and aligned to the laser. The current setup was to align the Cavity to a specific beam path that another laser could be aligned to. The main reason for having an alignment laser was because our diode laser had a bad mode structure and was low power, by using a Helium-Neon laser we are able to align to Cavity to a high finesse without the difficulty of having to deal with bad modes. Also note that M2 is a flip mirror and the there is another set of beam steering mirrors behind it for the diode laser, which is not shown.
+Our  setup former was configured as a Fabry-Perot Interferometer. This setup was used to verify that our Fabry-Perot Cavity was tuned and aligned to the laser. The current setup was to align the Cavity to a specific beam path that another laser could be aligned to. The main reason for having an alignment laser was because our diode laser had a bad mode structure and was low power, by using a Helium-Neon laser we are able to align to Cavity to a high finesse without the difficulty of having to deal with bad modes. Also note that M2 is a flip mirror and the there is another set of beam steering mirrors behind it for the diode laser, which is not shown.
 === Progress ===