Low noise position sensitive detector for optical probe beam deflection measurements

We demonstrate a novel electronic readout for quadrant photodiode based optical beam deflection setups. In our readout, the signals used to calculate the deflections remain as currents, instead of undergoing an immediate conversion to voltages. Bipolar current mirrors are used to perform all mathematical operations at the transistor level, including the signal normalizing division. This method has numerous advantages, leading to significantly simpler designs that avoid large voltage swings and parasitic capacitances. The bandwidth of our readout is solely limited by the capacitance of the quadrant photodiode junctions, making the effective bandwidth a function of the intensity of photocurrents and thus the applied power of the beam deflection laser. Using commercially available components and laser intensities of 1-4 mW we achieved a 3 dB bandwidth of 20 MHz with deflection sensitivities of up to 0.5-1 V/nm and deflection noise levels below 4.5 fm/Hz. Atomic resolution imaging of muscovite mica using FM-AFM in water demonstrates the sensitivity of this novel readout.

Section: Introductionmentioning

confidence: 99%

A high frequency sensor for optical beam deflection atomic force microscopy

Enning

Ziegler

Nievergelt

et al. 2011

“…It consists of a lens, two mirrors, and two photodiodes, and has been described previously. 21 The photodiodes are connected in parallel, with opposing polarities, thereby allowing a single low-noise current preamplifier ͑Stanford Research Systems Model SR570͒ to convert the photocurrents into a voltage which is proportional to the angular deflection of the probe beam. The amplified deflection signal is sent to a lock-in amplifier ͑Stanford Research Systems Model SR830͒, which can use the timing signal from the mechanical chopper as an external frequency reference.…”

Section: ͑1͒mentioning

confidence: 99%

Photothermal deflection spectroscopy of an aqueous sample in a narrow bore quartz capillary

Spear

Klunder

Russo

1998

Self Cite

Articles you may be interested inMid-infrared photothermal heterodyne spectroscopy in a liquid crystal using a quantum cascade laser An apparatus to perform photothermal deflection spectroscopy on liquid samples within a cylindrical capillary is described. A tunable dye laser, modulated by an optical chopper, serves as an excitation source. The resolution of a spectral absorption peak near 575 nm, of a 1ϫ10 Ϫ3 M Nd 3ϩ aqueous solution, demonstrates the effectiveness of the system. The sample is contained within a 75 m internal diameter quartz capillary, typical of those used for capillary electrophoresis. Across the middle of the probed section of the capillary, the magnitude of the resolved peak is 5.2ϫ10 Ϫ5 absorbance units. A helium-neon laser, focused to a 1/e 2 waist diameter of 40 m, provides an optical probe beam across the center of the sample, overlapping the excitation beam at an angle of 3°. Maximum signal-to-noise ratio is achieved with the apparatus when the excitation beam is modulated at a frequency near 205 Hz. The deflection responsivity of the probe beam at this frequency is 650 nrad per W of absorbed excitation radiation, with an internal noise level in the system of 0.6 nrad Hz Ϫ1/2 . The shot noise from probe beam radiation upon the photodiodes in the position sensitive detector exceeds noise from other sources. ͓S0034-6748͑98͒03506-0͔

dc and ac optical nulling bridges for sensitive transmittance measurements

Argueta-Diaz

Trejo-Valdéz

Garcı́a-Valenzuela

2000

We present a formal comparison between dc and ac nulling optical bridges. We consider their performance in monitoring over long periods of time (several minutes), small and slow (in a time scale of seconds) changes in transmittance of an optical component. We consider two fundamentally different ac optical bridges (OB), an amplitude modulated optical bridge (AM-OB), and a switching optical bridge (Sw-OB). For each OB, we derive a general expression for the minimum detectable change in transmittance (|ΔT|min), taking into account all the important noises. It is found that under optimum conditions the dc- and AM-OB have similar detection limits imposed by the 1/f noise of the photodetectors. It is shown that the Sw-OB can in principle overcome the 1/f noise of the detector and approach the shot-noise limit; however, it is sensitive to switching device imperfections (to first order in smallness), which could easily prevent achieving a detection limit below the 1/f noise. It is also shown that the Sw-OB has intrinsic advantages over the dc- and AM-OB for its use in remote sensors. It can eliminate more efficiently noise induced along the propagation of light from the sensing point to the photodetector. We conclude that when using low power optics (P⩽1 mW) and considering a bench instrument, the dc- and AM-OB can be used for a target resolution down to |ΔT|min≈10−5–10−6. The Sw-OB optical bridges should be chosen if |ΔT|min≈10−6–10−8 is to be attempted. In any case, strict conditions are to be met before considering approaching the detection limits imposed by electronic noises. These conditions are discussed in detail. In particular, atmospheric isolation will be needed in general below |ΔT|min≈10−4.