{"product_id":"thz-optical-chopper-model-300cd","title":"Optical Chopper, 5 Hz - 3 kHz, 102 mm Blade, Lock-in Reference Source","description":"\u003ch2\u003eOverview\u003c\/h2\u003e\n\u003cp\u003eAn optical chopper is a rotating slotted blade that periodically blocks a laser beam, converting a continuous signal into an amplitude-modulated one at a precisely known frequency. In a terahertz time-domain spectrometer the chopper sits in the pump arm and supplies the reference frequency that the lock-in amplifier demodulates against. This unit covers 5 Hz to 3 kHz as standard with a frequency stability of 0.1%, and can be extended from 15 mHz to 40 kHz with alternative blades.\u003c\/p\u003e\n\u003ch2\u003eWhy chopping is essential in THz-TDS\u003c\/h2\u003e\n\u003cp\u003eThe electro-optic sampling signal is a very small differential photocurrent riding on a large DC background, and it is buried in 1\/f laser noise, mechanical drift and detector noise. Chopping the pump beam moves the terahertz-induced signal to a well-defined carrier frequency, and the lock-in amplifier then rejects everything outside a narrow band around that frequency. Choosing a chopping frequency that is not harmonically related to the mains frequency, and not close to any mechanical resonance of the table, typically buys 40 to 60 dB of dynamic range.\u003c\/p\u003e\n\u003ch2\u003eBlade and mechanical design\u003c\/h2\u003e\n\u003cp\u003eThe 102 mm blade is photochemically etched rather than punched, so the slot edges are clean and the duty cycle is well defined; it is blackened to suppress back-reflection of the blocked pump beam into the beamline, and demagnetised so that it does not couple stray fields into nearby detectors. The open-frame head keeps the aperture accessible and lets the blade be positioned close to a beam waist, which minimises rise time and therefore modulation sidebands.\u003c\/p\u003e\n\u003ch2\u003eIntegration\u003c\/h2\u003e\n\u003cp\u003ePlace the chopper at or near a focus in the pump arm so the beam is fully blocked and fully passed in the shortest possible time. Feed the TTL reference output to the lock-in amplifier external reference input. Note that the chopper head, not the controller, is the element that occupies space on the optical table and must be included in the mechanical layout; the controller is normally rack- or shelf-mounted off the table.\u003c\/p\u003e\n","brand":"WaveQuanta","offers":[{"title":"Default Title","offer_id":48986395050138,"sku":"WQ-THZ-CHOPPER-300CD","price":0.0,"currency_code":"SGD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0734\/6156\/3546\/files\/WQ-THZ-CHOPPER-300CD-card-square_a309e4b8-0f93-4036-a4f2-71781513aebe.jpg?v=1786343962","url":"https:\/\/waveqvanta.com\/products\/thz-optical-chopper-model-300cd","provider":"WaveQuanta","version":"1.0","type":"link"}