1. Multi Frequency Laser Driver for Near Infrared Optical Spectroscopy in Biomedical Application Chenpeng Mu Department of Electrical and Computer Engineering, Drexel Univ, Philadelphia,PA, 19104

2. Introduction What is tissue spectroscopy? Near infrared spectroscopy system introduction. Driver design Gain, frequency response, linearity and noise System evaluation Optical property extraction Conclusion

3. Spectroscopy Absorption and scattering parameters of tissue are different with different wavelength of light. Frequency domain photon migration (FDPM) is used to extract optical properties of tissue(absorption and scattering). Photon penetrates tissue and penetration depth depends on modulation frequency. Tissue is multi-layer constructure, so multi-frequency should be used for better spatial resolution. GHz Frequencies MHz Frequencies Detector-1Detector-2Source-1 Fat Muscle 760850980

4. L aser D iode S ystem Broad Bandwidth Frequency Domain Instrument TURBID MEDIUM Network Analyzer (HP 8753ES) Sample channel RF source SP4T RF Switch LD4 980 nm LD1 680 nm LD 2 780nm LD3 830 nm 4 X N Optical Switch Source Fibers … N Amplifier Photodiode M X 1 Optical Switch … M Detector Fibers Turbid medium(tissue)

5. Optical link driver

6. Designed Active Laser Driver : AC simulation Frequency (MHz)Input current (mA)Output current (mA) 1005.63453.70 2006.94453.31 3008.69452.75 40010.66451.98 50012.73450.99 60014.86449.80 70017.01448.41 80019.18446.81 90021.33445.03 100023.47443.06 RF current is monitored RF current = 450mA

7. Designed Active Laser Driver : Amplitude Response and Phase Response Simulation vs. Measurement (A) (B) SimulationMeasurement Phase distortion < 1 degree Magnitude distortion < 1.5 dB

8. Noise measurement of Driver Calculation +3.3V Experimental Setup Laser Diode Driver

9. Laser Driver Laser Diode Model Photo Detector Optical System Performance: Simulation Hamamatsu Amplifier, G=34dB LD APD (Photodetector) Automated Network Analyzer (ANA) Optical transmitter (driver) TISSUETISSUE

10. Total Noise2.215uV Thermal Noise1.809uV RIN Noise1.183uV Shot Noise0.488uV Dark Noise0.011uV Thermal Noise is dominant. Optical Link Performance: simulation results

11. A B Phase distortion < 5 degrees Magnitude distortion < 2 dB Optical Link Performance: Experiment result Phase Magnitude

12. Extraction of Optical Properties: Transmission Model For N number of dipoles one can get the analytical solution for transmittance as: Calibration-corrected data are fitted with normalized theoretical transmittance to extract the optical absorption and scattering properties of the tissue.

13. Extraction of Optical Properties: Experiment Result Phantom ANA APD Laser Mount d PhantomExtracted F (MHz)  a (cm -1 )  s ’ (cm -1 )  a (cm -1 )  s ’ (cm -1 ) 1180.05100.05210.1 2260.05100.05410.5 3160.05100.0489.8 8380.05100.05811.0 9100.05100.05111.2 9640.05100.05710.6

14. Conclusion An active laser driver is developed for a broadband operation of four-color sources in near IR. A multi-frequency domain instrument is reported for near infrared light spectroscopy applications. High power (up to 1.2W) and high-speed (up to 1GHz) laser diode driver exhibited a flat frequency response. Extracted optical parameters  a and  s for phantom resembling breast tissue demonstrates the high accuracy of this measurement technique and extraction method.