Can you provide a bit more information about your set up? How are you generating your reference signal? How is the reference signal coupled to the photodetector output? When reading the photodetector with the DMM, are you reading 300 mV AC or DC? The output from your lock-in should be a DC signal that is proportional to the signal content that is equal in frequency and in phase with your reference signal.

Is your signal is AC or DC?

How do you modulate the signal? I assume, that you do modulate it.Otherwise, it just undermines the whole idea of the synchronous detection 

@ Ron, Cuong, Serge : I come to know about the Lock-In that it reads only AC signal. So one has to put a Chopper at the input end. Also for data acquisition one can take readings on computer by using some software like Labview. The cable to be connected is GPIB. Labview as well as GPIB, both are not available with us presently. I have made a RS 232 cable with one end 25 pin male and other 9 pin female to connect Lock-in with computer. But the problem is Lock-in is not detected though that cable by the computer. What shall I do.   

Also If I want to buy Labview, which suite shall I go for. As it is mentioned from basic to professional packages of Labview. I just want to control Lock-In.

You certainly do not need to purchase LabVIEW to write a program to run the SR830 through the RS-232 interface. You could easily write a nice interface with QT. I would highly recommend looking into open source programming options before making the investment in LabVIEW. I do use LabVIEW, I have the developer versions for Windows and Linux but I find that I more often than not, just write the programs in something open source since it's much easier to share with others that way.

Your computer will not "detect" that the SR830 is plugged into the serial port. You will need to query the serial port with the computer and see if you get a response from the SR830. The SR830 will not send any data unless it receives a command over the serial port. Make sure that the baud rate, data bits, stop bits, parity, etc. are set the same on your computers port and the SR830. You should be able to do basic communication tests with something like TeraTerm or CoolTerm.

Sir,

  Thank you. I understand what you are saying. But, we are still not able to read Labview through computer.  The RS 232 cable I have is self-made. We bought the connectors and cable separately and then soldered them according to the pinout diagram.

Then the 25 pin male was directly connected into the Lock-in, at the RS 232 (DCE) port. While the 9 pin female is connected to another connector (9-pin Serial to 10-pin Header) to further connect it into motherboard. The second connector image is attached below. Next, I use Cool Term software, which is showing that there is " no serial port " in the computer.

What shall I do.

You should first verify that the serial port is enabled in the computer BIOS. Are you sure the port you are using on the motherboard is RS232? I didn't think they even put them on mother boards any more but maybe you have an older computer. Once verified, you should be able to go into the OS to set up serial communication parameters in your hardware setup. This will change a bit depending on the OS. Once your OS recognizes the serial port, then you should see it in CoolTerm. Make sure that you have the correct interface chosen on the SR830. You will need to send the OUTX0 command to make sure that the SR830 sends it's communications output to the serial interface. You can send commands over both  GPIB and RS232, but the SR830 will only respond on the interface chosen with the OUTX? command (0=RS232, 1=GPIB). If you still don't see communication with the SR830, you should test the port with a loopback plug before proceeding. If the port works correctly with the loopback plug, then verify that that cable is wired correctly. There is often much confusion about how to wire cables between DTE and DCE, especially when using 25 and 9 pin on the same cable since Tx and Rx are reversed for the different pin counts. There are many more sites on the internet with confusing, incomplete, or just plain wrong information than not. B&B Electronics has very complete and accurate information. Try this link http://www.bb-elec.com/Learning-Center/All-White-Papers/Serial/FAQ-RS-232-Connections-That-Work.aspx

Sir,

  I changed the CPU with the one which is having COM port. Now serial port is being able to detect by the serial comm. software. But now I have to receive data from Lock-in. Can you please some more software names so that by using any of them I can detect Lock-In.

Thank you 

You can query the SR830 from CoolTerm. If you are unable to communicate with the SR830 via RS232 then you have either a cabling problem or a communication parameter problem. I think the SR830 uses 2 stop bits which is not the default for most serial configurations. With a loopback plugged into the computer serial port, type OUTX0 into your terminal program and hit enter. What is displayed in the terminal program? You should see the characters you type echoed back to you through the loopback. If not, you need to check your port configuration. One that works, connect your SR830 and type that line again followed by OUTX and hit enter. The SR830 should reply to the serial port which interface is selected. Then try ALRM and see if that returns data. once this is working, you can look through section five of the manual to see the other commands and queries. You can use Python, C, C++, QT, or just about any other programming language to make a UI for the SR830.

Sir,

Do you have any idea of optical power meters.

Sir,

  I used a server (standard 2008) to run Labview and connected Lock-in with it. I used both CoolTerm software and a software made on Labview to communicate with Lock-in. But no data is being received by OUTX0 or *IDN? or OUTX? commands. We checked the serial COM port on the computer, which is Fine. Is there a fault in the cable we made (25 pin male to 9 pin female) ?

Thank you

 I need to know how did you solve this problem . I am using lock in amplifier sr 530 and I have an error in the labview program . Did u made time delay in the program and how did u determine the bit length

Which type of error are you getting ?

The out readings in the programe for ch 1 and ch2 are different from that appear on the lock in amplifier screen.

They will be....since on the lock-in apmlifier the readings are in RMS values and on computer thru labview it is actual value.

But it fluctuates while the reading on the device is stable

You have to take the average of say 1000 readings at the output end.

What is the relation between ch 1 and ch 2 . What is the physical meaning

They are separate. They dont have any relation. You can take output from Ch 1 alone.

Have you solved this problem yet? It could be an issue with your time constant.