Attachment_A_-_Statement_of_Work_for_Beat_Note_Generating_Laser_System.docx

ATTACHMENT A – STATEMENT OF WORK FOR BEAT NOTE GENERATING LASER SYSTEM

BACKGROUND

The National Institute of Standards and Technology (NIST) Physical Measurement Laboratory (PML) Nanoscale Metrology Group (NMG) develops length measurement science and technology for advanced manufacturing nanotechnology and nanoscience, including time-varying length (i.e., motion). Measurement capability at this scale is critically needed, for example, by the semiconductor manufacturing industry for continued scaling and device integration, and for the innovation of new nanotechnology devices. The NMG disseminates the SI base unit of length down to the atomic scale, and the derived units of acceleration and acoustic pressure. Current focus areas include three-dimensional metrology of nanostructures including nanoparticles; micro- and nano-fluidic devices; and micro-electromechanical systems (MEMS). The NMG develops and applies interferometry methods for measuring displacement and length, and advanced microscopy methods such as scanning electron, atomic force, helium ion, and optical

PURPOSE

The purpose of this acquisition is to fulfill NMG’s requirement for a laser system that generates a beat note between two tunable diode laser frequencies that generates a system for the optical excitation of micro- and nanoelectromechanical systems (MEMS/NEMS) and materials. Optical excitation will be used to drive vibrations and acoustic waves through the photothermal effect and radiation pressure. The excitation will be used to probe the dynamics of these systems at frequencies as high as twenty (20) GHz.

SPECIFICATIONS

A laser system is needed that generates an optical beat note frequency that is the difference between the optical frequencies of two tunable lasers. The beat note is generated through interference between the two lasers and the beat note frequency must be tunable between 10 MHz and at least 20 GHz. This system will be used for the optical excitation of micro- and nanoelectromechanical systems (MEMS/NEMS) and materials.

Laser System Components The laser system must have the following components:

A. Lasers #1 and #2 B. Laser Controllers #1 and #2 C. Laser Linewidth Controller #1 D. Laser Linewidth Controller #2 E. Potassium Absorption Cell System E. Fiber Couplers

The necessary control electronics for running the lasers within the prescribed specifications shall be included. A digital controller is preferred over analog electronics but is not required

These components will be arranged as shown in Fig. 1.

A. Lasers #1 and #2 Lasers #1 and #2 shall be identical tunable diode lasers that are designed for exceptional wavelength stability. Anti-reflective coated laser diodes shall be used. The laser head shall include the required optics, mechanics, and piezoelectric elements for tuning the laser over the wavelength range described below. An optical isolator and fiber aligner for fiber output shall be integrated in the laser head. The two lasers shall be adjusted using coarse tuning to get the two frequencies to overlap and produce a beat frequency no greater than 20 GHz before delivery. Realignment of this overlap when necessary will be performed by NIST staff with the aid of a wave-meter.

1. Nominal wavelength: 766.5 nm

2. Nominal power: 80 mW

3. Coarse tuning range: 30 nm

4. Mode-hop free tuning range: 40 GHz

5. Typical linewidth: < 1 MHz over 5 microseconds

6. Beam polarization: Linear > 100:1

7. Fiber coupling efficiency: > 55%

8. Optical isolation efficiency: > 60 dB

9. Optical isolator transmission: > 80%

B. Linewidth Controller #1 Linewidth Controller #1 uses the feedback from the potassium absorption cell to lock Laser #1 to the spectral line at 766.5 nm. The controller shall be compatible with the absorption cell electronics and the laser main controller electronics. The controller shall use a proportional integral derivative (PID) control law for locking.

C. Linewidth Controller #2 This controller shall lock the beat-note frequency to a reference radio frequency (RF) signal supplied by NIST staff. This is done by mixing the beat note signal, or a frequency divided version of the beat note signal, with the reference RF signal. The resulting signal is used for feedback to lock the laser.

1. RF Mixer Bandwidth: from 10 MHz to 200 MHz

2. Control bandwidth: 10 kHz

3. Control law: PID

4. Beat note frequency stability: < 50 kHz

D. Potassium Absorption Cell System A fiber-coupled potassium absorption cell with photodetectors is required. Doppler-free and Doppler-broadened measurements shall be possible. Amplifiers and signal processing electronics shall be included. The absorption measurement shall be able to work with laser power below 500 microwatts. The combination of the absorption cell and laser controllers shall result in a linewidth better than 50 kHz.

E. Fiber Couplers The 1X2 coupler shall have a 99/1 splitting ratio and the 2X1 coupler shall be 50/50. All fiber shall be single-mode polarization maintaining fiber designed for 766.5 nm, approximately. The fiber connectors shall be FC/APC. Coupler length = 1 m.

System Process

Laser #1will be coupled into a 1X2 fiber coupler. 99% of the light will continue to the experiment and 1% will be sent to a potassium absorption cell system. The absorption cell is used to lock Laser #1 to an atomic spectral line at 766.5 nm. The signal from the absorption cell is sent to Linewidth Controller #1, which locks the laser to the spectral line. This provides a stable wavelength reference for the experiment.

Light from Laser #2 is coupled into one end of a 2X1 50/50 fiber coupler. The light from Laser #2 interferes with the Laser #1 in this fiber coupler, resulting in a beat-note frequency that is the difference between the two laser frequencies. The interfering beams are collimated and launched into our experiment. The resulting light is sampled with a beam-splitter and measured with a fast photodetector. The signal from the photodetector is sent to Linewidth Controller #2. This controller mixes the signal with a reference RF signal, thereby producing a feedback signal for adjusting the wavelength of Laser #2. The controller generates a corrective control signal, which is sent to the laser.

The system shall be able to control the beat note between 10 MHz and 20 GHz by adjusting the reference RF signal. It is expected that the RF mixer in Linewidth Controller #2 will not be able to function over this range. Therefore, the Contractor shall supply frequency dividers to provide a signal that is within the bandwidth of the mixer. The measurement of the beat note will be done with equipment supplied by NIST and will be sent to the Linewidth Controller #2. The beat note stability must be better than 50 kHz. The delivered system must be complete, provided by one vendor, and meet all specifications.

Fig. 1 Diagram of beat note generating laser system image1.png