Four-wave mixing

Strange four-wave mixing.

First problem was the auxilary signal. We didn't find real nature of it. Two guesses are either it is auxiliary four-wave mixing happening in the AOM crystal (or fiber). Or this is modulation of Toptica. To get rid of this signal, we had to install isolator in one arm of side pumps.

Four-wave mixing from cold atomic cloud

It is sensitive to alignment of side beams. After a long break it worth overlapping beam and the MOT with a camera.

The Raman signal we had got while were scanning the cavity.

Low amplitude four-wave mixing

We expect to have macroscopic signal, unfortunately up to this moment (7th of July) the highest amplitude was 1-1.5 mV on spectrum analyser (RBW= 5 MHz, VBW = 300 Hz). This corresponds to 1 nW of optical signal.

Pulsed regime

We are trying to switch off the MOT, for this we should send signals from arduino+DDS to:

• Repumper
• Cooling
• Magnetic coils
• Trigger signal to Spectrum analyzer

On top of this we need to lock the cavity.

Cavity lock

We made PDH (Pound-Drewer-Hall lock) lock for our cavity.

Additional noise from PDH

Nonperfect PDH lock behaviour causes huge noise on spectrum analyzer. The configuration is as follows we have two lasers going into the cavity: MBR+X (X = 80 MHz) and cavity lock laser, we do heterodyne measurement with MBR and MBR+X, spectrum analyzer is set to zero span.

Noise on homodyne

The

The

The problem we faced doing this is an increase of shot noise (the more accurate term will be just noise) in a homodyne detector. It happens when both cavity lock laser (yellow) and LO are present. And it is frequency sensitive, we had maximum around 3.6 GHz and 7.16 GHz, away from which (less then 100 MHz) it disappears entirely. If we perform the same experiment with Toptica noise appears @ 4.6-4.7 GHz. We think that MBR LO reflects from a photodetector window and travels all the way back into yellow laser, causes injection modulation of the laser. Relaxation oscillation?

We have sent yellow laser alone onto ultra-fast photodetector and detected a small side-band @ 3.8 GHz (see the picture), for the toptica a side-band

Thus we want to work with cavity lock laser @ 6.9 GHz

Strange four-wave mixing #2

We saw four wave mixing signal in the presence of two side pumps (Toptica-56, MBR+100) and yellow laser going through the cavity. The presence of atoms was essential. It fully covered our signal 4WM under.

Simplified pulsed regime

The first trial (12 Sept 2016):

1. lock yellow laser to MBR
2. get MOT continuous, in the program main loop if (0)
3. make a cooperativity high
4. switch off one coil and lock the cavity (minimum on spectrum analyzer)
5. switch on coils back on, run pulsed regime in the program if (1)

Auxiliary 4WM #3

Or stimulated Raman scattering. Appeared in the presence of atoms, yellow laser and MBR+80. The measurement was a regular heterodyne detection with a zero span on 80 MHz. Power of yellow laser was 300-400uW, MBR+80 had a few nW.

The Raman signal we had got while were scanning the cavity.

The Raman signal we had got while were scanning the cavity.

Pulsed regime 19 October

Trial #	Power MBR side [mW]	Power Toptica side [mW]	HWP MBR side	HWP Toptica side	QWP angle	Power toptica fiber	Peak FWM [mV]	Shot noise [mV]	FWM power [mV]
1	5.8	16.8	270	350	98	73u	5.493	2.428	3.065

Pulsed regime 21 October

Trial #	Power MBR side [mW]	Power Toptica side [mW]	HWP MBR side	HWP Toptica side	QWP angle	Power toptica fiber [uW]	Peak FWM [mV]	Shot noise [mV]	FWM power [mV]
2	1.455	12.6	?	?	?	16-17u

 

Things to do

1. Find out the reasons of four-wave mixing amplitude oscillations (when the cavity is locked). 2. Lock the MBR to Rb transition 3. Magnetic switch issue 4. How stable the locking for the PDH? 5. Mode matching of two side pumps on a camera 6. Further cooling of the atoms. 7.

 

 

 

5th of January 2017

Parameters:

Trial #	Power MBR side [mW]	Power Toptica side [mW]	HWP MBR side	HWP Toptica side	QWP angle	Power toptica fiber	Peak FWM pulsed [mV]	Shot noise [mV]	FWM power [mV]
1	1	15	257	334	102	80 u	12	?	?

Diameter of MBR side is 6-7 mm, the power of 4WM decreased in pulsed regime with time (down to 5-6 mV). If we switched back to continuous regime it returned to a high level (10-12 mV).

 

10th of January 2017

We observed a quantum 4WM, in the regime where we had 10 mV of continuous classical 4WM. And we changed MBR etalon offset from $\Delta =-0.26 GHz$ to $\Delta =-0.29 GHz$. We made the offset less, which means that the frequency of the laser becomes larger, and we are making MBR side more blue detuned. We had maximum of classical 4WM 30 MHz closer to resonance compared to quantum one.

To improve signal of classical 4WM we had to adjust:

1. move MOT along the cavity mode
2. align MBR side mirror
3. align Toptica side
4. adjust MBR power
5. adjust Toptica power (seems to be saturated after 18-19 mW)

 

Ideas for improvement, questions:

• 4WM through the cavity mode
• What gets wrong? why does sometimes power of the signal drops to half of it's value? Alignment/power?
• Is toptica still in 2 photon resonance?
• Is MBR side perpendicular to the cavity mode?
• Set proper detunings so we don't need to adjust after cooperativity check.
• Cooperativity dependence on 4WM power
• Хуй

 

 

23d of January 2017

Today we adjusted to have maximum cooperativity.

The detuning was

 

 

 

 

30th of January 2017

We saw 20 mV of signal in pulsed regime.

Trial #	Power MBR side [mW]	Power Toptica side [mW]	HWP MBR side	HWP Toptica side	QWP angle	Power toptica fiber	Peak FWM pulsed [mV]	Shot noise [uV]	FWM power [mV]	SG Toptica [MHz]	SG Cav lock [MHz]
1	3.5	15	262	330	190	15u	20	800	19.2	18.06	17.95

 

 

 

 

February 2017

Date #	Time	Power MBR side [mW]	Power Toptica side [mW]	Power toptica fiber [uW]	FWM Ampl. [mV]	SG Toptica [MHz]	Top. AOM [MHz]	Signal w0  (set on spectrum analyzer) [MHz]	MBR Offset Coop max [MHz]	MBR Offset max FWM  [MHz]	MBR Offset resonance (Side MBR blows away atomic cloud) [GHz]	MBR Offset peak 1 (4WM max nearest to resonance) [GHz]	MBR Offset peak 2 (4WM max nearest to resonance) [GHz]	Diff [MHz]	F transition	Toptica fiber power saturation [uW]	Comments
Feb 21	17:00	5	12	2.5	5.5	18.045	60	39									3 days off, balance detector
Feb 14					5										1	65
Feb 14		3.5	12	2		18.045									1
Feb 13		3.5	~12								3.04	3.00	2.93
Feb 13
Feb 8	17:18		~12		2.4	18.04	60		-450	-510				+60	1
Feb 8	17:28	5.5	~12		3.5	18.04	60		+320	+240				+80	1
Feb 8			~12			18.04	60	38.9	+370	+290				80	1
Feb 8	19:35		~12		2	18.05	60		+450	+380				70	1
Feb 8	20:30		~12		4.3	18.05	62	37	+540	+500				40	1
Feb 8	20:58		~12						-320	-350				30	2
Feb 10	13:40	2.2	12.3	1.7	2.4	18.05	60	40	+830	+810				20	1		The homodyne is balanced
Feb 10	15:40	3.2	11.8	2	2.6	18.05	60	40	+620	+600				20	1		Homodyne is rematched the locked cavity
Feb 10	15:40				2.9	18.045
Feb 10	15:40			48	8
		4			8.3
		4.7			8.3
		5.5			8.3
		6.5			8.6
		6.5			8.8
		7.5			8.5

 

Classical FWM

27 February

Other Parameters are Toptica pump=12.5 mW, MBR pump = 15 mW.

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Signal vs Toptica seed power

 

1 March 2017

Other Parameters are Toptica pump=12.5 mW, MBR pump = 14.4 mW.

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FWM signal vs Toptica seed power

14 March 2017

EIT measurement. We send Toptica side (Toptica - 60 MHz) and weak (MBR + 81 MHz) through the cavity mode. Less than 1 nW of MBR, or 950 uV on the spectrum analyzer. MBR +81 is set to be in resonance with atoms, then topica side is added. The optimum value for toptica side frequency makes 18.14 MHz reference for the phase-lock loop. This reference sets difference between MBR and Toptica: \omega_{toptica} - \omega_MBR =18.14*384 MHz= 6965.76 MHz

Collinear 4WM: February 2018

• (23.02) We have measured FSR of our cavity. For this we have sent yellow laser through AOM, leaving only the blue sideband -96 MHz, together with MBR laser and observed the brightest spot on the rear mirror of the cavity on the camera. Optimum frequencies of reference signal were $w_{RF} = 283.5-284 \,\text{MHz}$, which corresponds to frequency difference between YL and MBR (multiplied by 24, N=48, R=2): $\Delta= \omega_{YL}-\omega_{MBR} = 6804-6816 \,\text{MHz}$. Taking into account AOM frequency shift we conclude that $\mathbf{2 \text{FSR}} = \omega_{MBR} - (\omega_{YL}-96\,\text{MHz}) = \Delta+96 \,\text{MHz}=\mathbf{6900-6912}\,\text{MHz}$. (refinement: on the wavemeter $\lambda_{YL}>\lambda_{MBR}$)
• (23.02) Second observation was that in the far field reflected cavity mode looks funny, probably due to interference with reflected field which doesn't enter the cavity.
• Let's set the experiment as Toptica straight being a pump, and Toptica $+\Delta_{SB}$ serves as a seed in resonance with Stokes. We can find Toptica SB (sideband frequency) (AC-stark shift is not included) $\Delta_{SB}$ = 2FSR-HFS=(6900-6912) - 6834.6826 = 65.3-77.3 MHz. We modulate Toptica to
• We look for the signal on spectrum analyzer on the frequency $\delta_{AS}= \Delta_{Y.AOM, MBR} -\Delta_{SB}=$115-(65.3-77.3) =50 -38 MHz
• We pick detunings for the two pumps from the condition $\Delta_{B}+\Delta_{R}=\Delta_{SB}$. Lower bound: $\Delta_{B}$=35 MHz, $\Delta_{R}$ =30 MHz. Thus AOM for MBR spectroscopy (same yellow) is $\tilde \Delta_{Y.AOM, MBR} = \Delta_{Y.AOM, MBR} - \Delta_{B}$ = 115-35 MHz=80 MHz
• What should be set for PLL $\omega_{top} = \omega_{MBR}+\tilde \Delta_{Y.AOM, MBR}+HFS -\Delta_R$=6884.7 or $\Delta_{RF, top}$=:96 = 71.7156 MHz. Negative slope on PL box.
• PLL box for the yellow laser $\mathbf{\Delta_{PLL, YL}}= 2FSR-\Delta_1+\Delta_R-\Delta_{B.AOM}=$ 6900-80+30-80=6900-130=6770 MHz or $\mathbf{\Delta_{RF, YL}}$=:96 = 70.52 MHz. Negative slope on PL box.
• (26.02) We have locked Rode&Schwarz to DDS, we found that set_frequency(10e6) doesn't give exactly 10 MHz(300 Hz error). Probably program error

Yellow laser frequency	Toptica lock frequency	Toptica modulation freq	2 FSR
$\mathbf{\Delta_{PLL, YL}}=6770$ MHz	$\omega_{top} = 6884.7$ MHz	$\Delta_{SB}$=65.3-77.3	2 FSR=6900-6912 MHz

10 July 2018

We realized injection locking of toptica laser with modulated MBR beam. We added AOM into Toptica pump path. We work in the regime, where we have 20 microW of each pump before the cavity.

12 July 2018

We checked FSR, and we get 2FSR= 6893-6902.6 MHz

19 July 2018

We checked FSR, and we get 2FSR= 6893 MHz, locked cavity 8 uW entering cavity, 180 nW going to a detector.

Locking frequency:

1) MBR+50=Top-20-6893

Top-MBR=6963(EOM frequency)

2) MBR+50-6893=YL-88

MBR - YL = 6755 (6755:96=70.36458)

24 July 2018

Latest result of 4WM and Raman signal from two homodyne detectors.