1. Laser Frequency Stabilization
REU Summer 2006
Margot Phelps
Layra Reza
Mentor:Dr. Deborah S. Jin

2. Outline Using an external cavity diode laser
stabilizing the frequency of the laser
How lasers are used in the experiment
Controlling the laser
Interpreting error signals

3. Building the laser External cavity diode lasers
Diode wavelength=767 nm.
Here we are collimating the beam.
Each built an external cavity diode laser

4. Threshold curves Helps characterize the behavior of the laser
The threshold is where the laser begins to lase
Lasing means that the laser is producing a coherent beam, all its photons are in the same quantum state
Important in case of suspected laser failure
After you have built the laser you measure how much power you get out of it per milliamp.This is the threshold curve. Get rid of blue and fix legend, and say laser failure instead of death. Say what the threshold means, what is lasing, compare to BEC, same quantum state(photons) instead of a white light bulb, explain stimulated emission,TIMESCALE frequency changes overtime, slow and fast drift of frequency, delta function, gaussian. Make same colors, same shapes.EXPLAIN AXES REALLY WELL

5. Beatnotes
Lasers drift in frequency over time. Instantaneously, they are lasing at one frequency, but a time averaged signal shows something different.
Reasons for drift: Electronic noise,Temperature drift,Air flow
Want the laser’s lineshape to be <1MHz
Replace slide with ones that explains the frequency drift over time, put a few delta functions etc. show optics setup that results in a beatnote, get a diagram, the beat is the difference not the center, width not how close signals are in freq, look mathematically what beatnotes do

6. Why is Frequency stabilization important?
Laser imaging needs the laser to be at the right absorption wavelength for the atoms
shadow image
The zero on the x axis represents the resonant frequency, at a wavelength of 767 nm. The y axis is in peak optical density, which is a measure of the density of the atom cloud.The farther away from resonance the laser is the less atoms there are in the cloud. Since the width of the entire cloud is 6 MHz, we want our laser to have a lineshape of <1 MHz so that we can hit the resonance most effectively, and stay on it.
Zero equals resonant frequency
processed image
(shows atom density)
The lineshape of the laser must be >1MHz

7. Feedback Loops Controlling the laser: A feedback loop or “servo” laser
Frequency
Measuring Device
laser
Laser light
Explain error signal better, as freq changes, signal gets bigger etc.REMEMEBR WHAT A GRATING IS. Explain error signal, with a graph, error signal is voltage, goes up or down with frequency, zero wrt electronics, fix laser light label from the servo. Instead of error signal put freq measurement device and then error signal coming out of it feedback loop instead of laser feedback
Error signal
servo
Electronic signal

8. Saturated Absorption Widely used setup for laser stabilization
Wide Doppler dip, with extra peaks
Scanning laser frequency to find peaks
saturated absorption peaks at 767 nm for 40K.
sidelocking to reduce frequency drift
photodiode output voltage(mV)
Introduce slide as:This is how we measure freq. Can be used as an error signal(already is an error sig) for sidelock. Needs to be better than 1MHz, beatnote needs to be smaller than a MHz. Explain axes of pic
Need key facts about sat abs, write on axes, voltage output of photodiode, explain Doppler broadening, need scale, all absorption lines, how wide is the broad part. Get the sat abs lines, better than Doppler because the slope is greater, trying to lock better than a MHz, these are as wide as a GHz
Remind people that the axis is freq., deliberately scanning to look at the lines
Frequency(Mhz)

9. Locking to the Peak
Frequency modulation : the wavelength of the laser is scanned across the atomic transition, and the wavelength modulation is seen as varying amplitude modulation.
Intro as: Problem with sidelock is that you might be very far away from resonance, more than 6 MHz, better to lock on the peak , chance title to peak locking, this is what it is for, closer to resonance, slope at 0 for resonance. So for peak locking you need a different error signal.
The ratio of AM to FM versus the laser frequency results in this derivative signal
This signal is at zero when the laser is on resonance
Use this as the error signal for peak locking the laser

10. The Challenge Saturated absorption lines Sidelock Peaklock
Pros
Don’t modulate the frequency of the laser itself
Cons
Might be too far away from the resonant frequency
Peaklock
Closer to resonance
Have to modulate laser frequency
Derivative signal
Simplify this slide, problem is you can either sidelock or peaklock, we want to do both, lock to peak and then use sidelock, under sidelock put pros no modulation on laser freq, opposite for peaklock and cons, title is the Challenge.
FM Spectroscopy
We want:
A way to combine the sidelock and peaklock to eliminate the drawbacks of both methods

11. Summary Importance of laser frequency stabilization
Some widely used methods
Room for improvement: other methods could prove to be better than those in use.